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: I6adc16da855835f9f518f8967628e5d0135c789b