Clean up and fix skia-RenderEngine tests

Skia-RenderEngine tests were accidentally exercising solely
GLESRenderEngine, so the wrong backend was being used. This patch:
* Correctly enables Skia-Renderengine tests
* Adds a downcasted pointer to GLESRenderEngine to access test-only
methods so that there's no loss of test coverage
* Disables Skia tests that rely on those GLES-specific test-only
methods, since those tests were behavior-specific anyways
* Correctly destroy the Skia RenderEngine instance
* Add fixes for the remaining failing tests. Fixes include:
** Fix SkImage caching to...actually cache
** Forcing the opaque bit must occur on the SkShader, rather than on the
SkColorFilter, so that the alpha channel is not sampled when computing
the src colors.
** When only a color transform is applied but color management is
enabled, converting between RGB primaries and linear XYZ reduces down to
an identity matrix, which avoids clamping artifacts in the shader.
** Apply source dataspace properly when there are solid colors
** Support for black clearRegions for SurfaceView

Bug: 173416417
Test: librenderengine_test
Change-Id: I530441a80039c7807931985665892017e4c48e76

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