surfaceflinger: persist.sys property for 16bpp transparency

new property: persist.sys.use_16bpp_alpha

Drop the USE_16BPPSURFACE_FOR_OPAQUE define which is now obsolete
with the CMParts performaince option :
    http://review.cyanogenmod.com/8440



Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>
Change-Id: Id9fca57fbd9c3f225326cbd77ce4fc053c9da002