surfaceflinger: Improve dithering method (1/2)

Some devices have blur effect while opening apps.
This fix adds an option(2) for "Fix blur effect".
Dithering is always enabled when persist.sys.use_dithering=2

Conflicts:
	services/surfaceflinger/SurfaceFlinger.cpp
	services/surfaceflinger/SurfaceFlinger.h

Change-Id: Id96de20160d6b2c80d6f7394e937ef64536ad87b