wm: recycle bitmaps immediately in TaskSnapshotPersister

Bitmap created in TaskSnapshotPersister are very short lived and
shouldn't be left around in the Java heap.

Test: boot, switch apps, works
bug 117795621

Merged-in: I4b5e0db50c2b7adaa71cb0d22535c1b37c7523e8
Change-Id: I4b5e0db50c2b7adaa71cb0d22535c1b37c7523e8