Fix NPE when clicking on a notification with >= 2 messages on SDK < 16.

Seemingly the intents in the task back stack got confused when the
pending intent was updated in those versions (the Accounts intent was
delivered to MessageList). Avoid that by not updating the current
intent, but dropping the old one.

Fixes issue #4955.

src/com/fsck/k9/controller/MessagingController.java

@@ -4939,7 +4939,8 @@ public class MessagingController implements Runnable {
         }
 
         builder.setContentIntent(stack.getPendingIntent(
-                account.getAccountNumber(), PendingIntent.FLAG_UPDATE_CURRENT));
+                account.getAccountNumber(),
+                PendingIntent.FLAG_CANCEL_CURRENT | PendingIntent.FLAG_ONE_SHOT));
         builder.setDeleteIntent(NotificationActionService.getAcknowledgeIntent(context, account));
 
         // Only ring or vibrate if we have not done so already on this account and fetch