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