Prefer 16bpp surface formats

Adds a property to use RGB565 surface formats for the opaque
surface default, enabled by default.

Change-Id: Iac9bbc8292a863e2ab86093df3ecf218e4ec61e1

Conflicts:

	services/surfaceflinger/SurfaceFlinger.cpp
	services/surfaceflinger/SurfaceFlinger.h