BackgroundManager fixes.

- Fix failure to fade out when a null bitmap/drawable is provided.
- Fix flashing caused by activity transition animating alpha on the window background.
  The layer drawable must apply its alpha to the layer composition of the child drawables.
- Fix case where BM was released then on resume the app provides a background before
  the background fragment is resumed.

b/19699049
b/19689219
b/17924621

Change-Id: I6d246badaf1d76682e602c2ba382d44290357665
1 file changed