Fade in/out taskbar bg alpha when unstash/stashing.

This prevents the case where we are drawing both the taskbar bg
renderer and the stashed handle at the same time.

Fixes: 287245176
Test: go home/overview, open/close notif shade, stash/unstash in app
Change-Id: Ib025a0cf1ad5fd8a9afd079aa50210f3b2388d8e