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

Conflicts:

	services/surfaceflinger/Layer.cpp
(cherry picked from commit f4c511eadf2450f1764617245205ffb940b710f8)
(cherry picked from commit 98d7ecf2a82fe6c2a82285b64ea9a8d8da443e9a)
(cherry picked from commit 2dcc6c014c7593d4d18667299718832a339905e3)