Support Alpha8->RGBA bitmap copying

Skia recently dropped support for this conversion.  So this adds
an implementation to the framework.

This also unifies framework behavior to treat Alpha8 as
implicitly black when it must be converted to a color.

Test: CTS

BUG:36202415

Change-Id: Ia141733387e80f7e61bae472e632cdfbac67b2e0