Fix HWUI/Skia Gradients to premultiply the colors prior to interpolation

This is fixed in Skia by passing the appropriate flag when the shader is
generated.  The fix in HWUI is to reverse the premultiplication and
interpolation steps.

Test: bit CtsUiRenderingTestCases:.testclasses.ShaderTests
Bug: 34323783
Change-Id: I3417141949f62fcc696b6d8213a4b446d7d0cbf8