Clear sendingToBottom when animation is complete.

Was counting on moving the app to the top to clear the flag
indicating that the app was being sent to the bottom. Since this
did not always happen the sendingToBottom flag was occasionally
left set. In this case the focus was skipped for that app and
consequently input was never propagated to it.

This fix clears the sendingToBottom flag each time the app
animations are completed.

Fixes bug 6691421.

Change-Id: I6f851dc5bedca95182db8490d87c876a71ad5fde