When app dies, destroy snapshot

Also destroy snapshot when we remove the AppWindowToken.

Test: runtest frameworks-services -c
com.android.server.wm.SnapshotCacheTest
Test: Open app, go home, kill app, make sure snapshots are
destroyed.

Change-Id: I532c2d7499a86164175f9fcbc8b77c6eb6bfeae6
8 files changed