libui: add ability to force a framebuffer format for EGL's use

This does not actually change the framebuffer format. It merely
fakes this format to surfaceflinger so that when it creates
framebuffer surfaces it will use this format. It's really a giant
HACK to allow interworking with buggy gralloc+GPU driver
implementations. You should *NEVER* need to set this for shipping
devices.

Change-Id: I03eeb5b4d72838ef219df386ecc489fc20ab9cc7
Signed-off-by: Dima Zavin <dima@android.com>