Add API to allow overriding a gainmap in BitmapShader

Fixes: 296482289
Test: atest android.uirendering.cts.testclasses.GainmapTests -- --template:map preparers=template/preparers/feature-flags --flag-value core_graphics/com.android.graphics.hwui.flags.gainmap_animations=true
Change-Id: I1984a625ff3740e8fffd4d1ffc6f16132dfc6788