Use alpha instead of invisible for views below shelf to skip redraw

Also skip this for heads up notifications

Fixes: 193912541
Test: treehugger
Change-Id: I3c81611696f7918a0f18d4c503b80a6b78ed2c43
(cherry picked from commit ef12c7a2b208d0828d2eefa5f9f49941b23c4b0b)
1 file changed