Make sure gradients can be composed with alpha value

If a gradient is used as a color, and a separate alpha value is defined
for that color, we must make sure the two are combined.

Bug: 122260583
Test: RenderTests.testGradientAlphaDrawable added
Change-Id: I0cddd6e62a822ef0f772303afd55388e55649ace
(cherry picked from commit 1079f327ca750e56fdc1faaa2115a6798c3675be)