Implement overlay window for Taskbar EDU and All Apps.

Having EDU in the same window as Taskbar causes it to be above All Apps
and other system views such as the notification tray. This change
refactors the existing All Apps window to accomodate more AFVs so that
EDU can also exist in it.

Demo: http://shortn/_Qcki3gwvf7

Test: Manual
Fix: 232177330
Change-Id: I1bef31d798041a90a0c3e033e71be63898fa5fbc