Optimizations for current wallpaper preview

Cache bitmaps more aggressively in BitmapCachingAsset, and
CurrentWallpaperAsset.
Also make WallpaperConnection and WallpaperSurfaceCallback not depend
on Activity

Bug: 184111918
Test: manual
Change-Id: I077f98e9a65759e32fd1896c67fe241ee8f68d1a
5 files changed