Support forcing all screenshots into a CPU consumer

Looks like a full GPU path is less efficient on some GPU
drivers that we're still using, and CPU is reliably faster...
(there's probably a locking condition going on somewhere, this
needs to be looked into)

Change-Id: I8878796a117d65bf2324507cf8755cadce49f6dc
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