bootanimation: allow using RGB565

RGB565 uses considerably less memory, and together with texture
cache, it allows us to offer a really smooth bootanimation on
not so powerful devices.

Signed-off-by: Andreas Blaesius <skate4life@gmx.de>
- changes for Android 5.0

Change-Id: Icbd6754cc608693c007ac92428e4aa711c32a59c