Check if returned surface params are null before adding them for surface transform

We call finish recents animation before app switch animation finishes. When we finish, the live tile params are not cleared up.

This change takes care of
- clean up recents animation targets when it's finished
- do a sanity check on live tile surface params before adding them to the surface transform array

Fixes: 143889142
Test: Swipe up to Overview in live tile mode, switch to a different task, and make sure it doesn't crash anymore
Change-Id: I089cf0a97744461d1fec4d0c97e0c3c6c1049ae3