Fix Floating window bug

1. any app in recent apps + launch floating app in home screen
2. close floating app
3. last app in recent apps will launch and close in 0.5 sec

Change-Id: I6fa8605b431ae8f82ca6be34a8fc4b9cab407c9a
Make sure we clean up

pruneDeadThumbnailFile need to close its cursor
in a finally block to avoid leak resouces.

Change-Id: I2646b91970e47ee80a721a2452215b7cae149d10