Cache wallpaper bitmap to avoid re-loading on rotation

Screen rotations force static wallpapers to get recreated. One of the things
that happens is that the underlying bitmap resource is loaded. This can be quite
expensive for large bitmaps (which is the case on large-display devices).
A simple optimization is to retain the bitmap in the wallpaper process, to avoid
this re-loading step. We still re-draw and re-upload the texture, but at least
we don't re-load the thing.

Issue #7324823 Manta wallpaper decode performance is atrocious

Change-Id: I0748e275a55992d13704a7dec5910d2dbdc9e2a4