Keep persistent Taskbar visible over Taskbar All Apps.

Additionally, this CL fixes some touchable insets issues with the
overlay window. Dragging and dropping is now reliable from taskbar while
All Apps or EDU is also open.

Change-Id: Ia612c1b5266a8fb2b7005a773fbd2b00ec4824db
Bug: 279465803
Bug: 275890048
Bug: 264463659
Flag: PERSISTENT_TASKBAR_OVER_TASKBAR_ALL_APPS (Teamfood)
Test: Taskbar is functional over All Apps and EDU.