Add BackgroundManager api for specifying the theme drawable.

Allows an app to avoid setting this drawable in the theme and thereby
skip the overhead of decoding the image when the activity starts,
which now happens commonly because leanback recommends different
themes for browse and details.

b/19991532

Change-Id: I0f5c73d91efd35a09c76d1bf87a184d1deb47f09
1 file changed