Enable wide color gamut rendering

When wide color gamut rendering is requested, hwui will now
use an rgba16f scRGB-nl surface for rendering. This change
also fixes the way screenshots are handled in the platform
to behave properly with wide gamut rendering.

This change does not affect hardware layers. They also
need to use rgba16f scRGB-nl; this will be addressed in
another CL.

Bug: 29940137
Test: CtsUiRenderingTestCases, CtsGraphicsTestCases

Change-Id: I68fd96c451652136c566ec48fb0e97c2a7a257c5