Auto-stash taskbar when locking device

This greatly reduces the the jank of the taskbar being shown above the
lockscreen.

Before: http://shortn/_PhXhkWR73t
After: http://shortn/_MC3ZLGHp4o

Bug: 264604213
Test: manual (http://shortn/_ISEXThUFM2), tapl
Change-Id: Ib1c5dbaafd4d0e1d69fbc03d908632e0275c8a0b