Use RGB565 surface for wallpaper

The framebuffer only uses 16bpp on most legacy devices anyway, so this
does not result in worse quality. It saves a lot of GPU fillrate and
memory, though.

Change-Id: If19902500d615291c4362f03d26d9cee2d4e684e
Prefer 16bpp formats for opaque bitmaps

Configure BitmapFactory to prefer RGB565, which will make it
use that format for all opaque assets loaded by it with standard
settings. Toggle dithering flag in strategic places and reenable
16bpp drawing cache, to speed up software UI rendering.

This generally helps with reducing memory consumption on legacy
devices, and usually does not result in reduced visual quality.

Change-Id: Id1b7e0529461d23213e31c25b74d4da5d08f0855