BackgroundManager: fix ThemeDrawable leak

Each activity gets different copy of Bitmap for ThemeDrawable.
So we should free theme drawable during release().  This wasn't
a problem before when all activities use same Leanback theme
and we weak-reference-cache last theme drawable by id.  In latest
leanback lib, browseactivity and detailsactivty are using
different themes, the weak-reference-cache doesnt kick in.

Move ThemeDrawable initialization into lazyInit().

Little cleanup: no need to hold mThemeDrawable and never use it.

Bug 19494763

Change-Id: I1093b88c3990c20fa5b8f5695c1e0368450a589e
1 file changed