Properly convert A8 -> RGBA_8888 gainmap contents

When doing the fallback from alpha8 to rgba8888 when
alpha8 gralloc buffers are not available, we need to convert
the contents as well to ensure that the gainmap shader
processes the correct channel(s)

Bug: 356778382
Test: silkfx w/ hardware allocation; updated uirendering GainmapTests
Flag: EXEMPT bugfix
Change-Id: I4bc571cd9e1f7715a1e269e516ce2678df297274