[SurfaceFlinger] Add GPU protected content support.

Switch to protected context when a layer with protected contents presents.
Previously we black out layers with protected contents, now that we add support
of protected contents in GPU composition, we can turn it on. In screenshot
cases for Recents, we still black out layers with protected contents.

BUG: 35315015
Test: Build, flash and boot
Test: Watch Youtube movie, verifed by force GPU composition.
Change-Id: I556bdd6f82c06c3d54ce62fab06a8c6f47599dcf