surfaceflinger: Allow gpu to render widevine level3

Allow gpu to render widevine level3 but keep
blocking screen shots.

CRs-fixed: 428647,424769,421767

Change-Id: I914232a062acbb7b17901dbf2b414973230e59d9
frameworks/native: fix copy back dirty region

Copyback dirty region logic does copyback,
even when its not necessary causing 2ms delay.
Fix the logic to copy back only what is necessary.

Change-Id: I52de68258ac9f87d704ee5401f93417805fa6773