DO NO MERGE -- Revert the following two commits as they're causing flickering

When double tapping Recents, need to reinvestigate.

b/30831873
b/30790402

Revert "Clear WS.mDestroying on AWT.clearAnimatingFlags"

This reverts commit c2661e52.

Revert "Some fixes for transition animation selection"

This reverts commit 73e9bc3f.