Use snapshot alpha for layers

Removes mMultipliedAlpha, using the snapshot alpha for all
non-overlapping display list alpha control.

Additionally, fixes opacity issues where children of
hasOverlappingRendering=false displaylists (both hw layer sublists and
other sublists with hasOverlappingRendering=false)

Change-Id: I1d0e561a4d4965d06c2a6827c7a31e6dbf731b86