Improve BACK button handling for MessageView

Remove memory leak from referencing MessageView context from the
Intent that is created to go back to MessageList. MessageView is no
longer hardcoded to go back to MessageList, it instead uses an Intent
given at creation to get back to the originating Activity.

Try our best to restore the MessageList in its previous state when
"Manage BACK button" option is enabled:
Since MessageList lives in its own task, we look for the previous
active task and check whether its top activity matches it. If it does,
we just finish MessageView and Android will automatically restore the
previous task. If it doesn't, we launch the originating Intent (and
MessageList state will be lost).

If option is off, we get the regular Android behavior: got back to the
previous screen, whenever it's the MessageList or another application
if the user long-pressed HOME.

The consequence of this is the need for a new permission in order to
check the previous active task: android.permission.GET_TASKS

AndroidManifest.xml

@@ -26,6 +26,10 @@
     <uses-permission android:name="android.permission.VIBRATE"/>
     <uses-permission android:name="android.permission.WAKE_LOCK"/>
     <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
+
+    <!-- In order to properly manage the BACK key, we do some check on the running tasks -->
+    <uses-permission android:name="android.permission.GET_TASKS" />
+
     <uses-permission android:name="org.thialfihar.android.apg.permission.READ_KEY_DETAILS" />
 
     <permission android:name="com.fsck.k9.permission.READ_ATTACHMENT"

src/com/fsck/k9/activity/MessageList.java

@@ -1070,7 +1070,7 @@ public class MessageList
             MessageReference ref = message.message.makeMessageReference();
             Log.i(K9.LOG_TAG, "MessageList sending message " + ref);
 
-            MessageView.actionView(this, ref, messageRefs);
+            MessageView.actionView(this, ref, messageRefs, getIntent());
         }
 
         /*

src/com/fsck/k9/activity/MessageView.java

@@ -1,5 +1,7 @@
 package com.fsck.k9.activity;
 
+import android.app.ActivityManager;
+import android.app.ActivityManager.RunningTaskInfo;
 import android.app.Dialog;
 import android.app.ProgressDialog;
 import android.content.Context;
@@ -34,6 +36,7 @@ import java.util.*;
 public class MessageView extends K9Activity implements OnClickListener {
     private static final String EXTRA_MESSAGE_REFERENCE = "com.fsck.k9.MessageView_messageReference";
     private static final String EXTRA_MESSAGE_REFERENCES = "com.fsck.k9.MessageView_messageReferences";
+    private static final String EXTRA_ORIGINATING_INTENT = "com.fsck.k9.MessageView_originatingIntent";
     private static final String EXTRA_NEXT = "com.fsck.k9.MessageView_next";
     private static final String SHOW_PICTURES = "showPictures";
     private static final String STATE_PGP_DATA = "pgpData";
@@ -62,6 +65,12 @@ public class MessageView extends K9Activity implements OnClickListener {
         HAS_SUPER_ON_BACK_METHOD = hasOnBackMethod;
     }
 
+    /**
+     * If user opt-in for the "Manage BACK button", we have to remember how to get back to the
+     * originating activity (just recreating a new Intent could lose the calling activity state)
+     */
+    private Intent mCreatorIntent;
+
     private SingleMessageView mMessageView;
 
     private PgpData mPgpData;
@@ -267,7 +276,20 @@ public class MessageView extends K9Activity implements OnClickListener {
         // or later, or by the code above on earlier versions of the
         // platform.
         if (K9.manageBack()) {
-            MessageList.actionHandleFolder(this, mAccount, mMessageReference.folderName);
+            final ActivityManager activityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
+            // retrieve the current+previous tasks
+            final List<RunningTaskInfo> runningTasks = activityManager.getRunningTasks(2);
+            final RunningTaskInfo previousTask = runningTasks.get(1);
+            final String originatingActivity = mCreatorIntent.getComponent().getClassName();
+            if (originatingActivity.equals(previousTask.topActivity.getClassName())) {
+                // we can safely just finish ourself since the most recent task matches our creator
+                // this enable us not to worry about restoring the state of our creator
+            } else {
+                // the previous task top activity doesn't match our creator (previous task is from
+                // another app and user used long-pressed-HOME to display MessageView)
+                // launching our creator
+                startActivity(mCreatorIntent);
+            }
             finish();
         } else if (HAS_SUPER_ON_BACK_METHOD) {
             super.onBackPressed();
@@ -328,20 +350,35 @@ public class MessageView extends K9Activity implements OnClickListener {
     }
 
 
-    public static void actionView(Context context, MessageReference messRef, ArrayList<MessageReference> messReferences) {
-        actionView(context, messRef, messReferences, null);
+    public static void actionView(Context context, MessageReference messRef, ArrayList<MessageReference> messReferences, final Intent originatingIntent) {
+        actionView(context, messRef, messReferences, null, originatingIntent);
     }
 
-    public static void actionView(Context context, MessageReference messRef, ArrayList<MessageReference> messReferences, Bundle extras) {
+    /**
+     * @param context
+     * @param messRef
+     * @param messReferences
+     * @param extras
+     * @param originatingIntent
+     *            The intent that allow us to get back to the calling screen, for when the 'Manage
+     *            BACK' option is enabled. Never {@code null}.
+     */
+    public static void actionView(Context context, MessageReference messRef, ArrayList<MessageReference> messReferences, Bundle extras, final Intent originatingIntent) {
         Intent i = new Intent(context, MessageView.class);
         i.putExtra(EXTRA_MESSAGE_REFERENCE, messRef);
         i.putParcelableArrayListExtra(EXTRA_MESSAGE_REFERENCES, messReferences);
+        i.putExtra(EXTRA_ORIGINATING_INTENT, originatingIntent);
         if (extras != null) {
             i.putExtras(extras);
         }
         context.startActivity(i);
     }
 
+    @Override
+    protected void onNewIntent(final Intent intent) {
+        mCreatorIntent = intent.getParcelableExtra(EXTRA_ORIGINATING_INTENT);
+    }
+
     @Override
     public void onCreate(Bundle icicle) {
         super.onCreate(icicle, false);
@@ -381,7 +418,10 @@ public class MessageView extends K9Activity implements OnClickListener {
         mMessageView.initialize(this);
 
         setTitle("");
-        Intent intent = getIntent();
+        final Intent intent = getIntent();
+
+        mCreatorIntent = getIntent().getParcelableExtra(EXTRA_ORIGINATING_INTENT);
+
         Uri uri = intent.getData();
         if (icicle != null) {
             mMessageReference = icicle.getParcelable(EXTRA_MESSAGE_REFERENCE);