Made Accounts a singleTop activity

Now, returning to the app using the app icon will reuse the existing
Accounts activity. This will skip the startup logic in onCreate().

AndroidManifest.xml

@@ -67,6 +67,7 @@
         <activity
             android:name="com.fsck.k9.activity.Accounts"
             android:configChanges="locale"
+            android:launchMode="singleTop"
             android:uiOptions="splitActionBarWhenNarrow"
             android:label="@string/app_name">
             <intent-filter>