Better support transient and persistent taskbar in taskbar's overlays.

- Transient taskbar nav threshold now works in overlays.
- Delay closing overlay to transient app-window threshold if necessary.
- Persistent taskbar no longer stashes for EDU overlay.
- EDU overlay provides enough bottom padding for transient and
  persistent taskbar.

Test: Manual
Bug: 217261955
Change-Id: I2ae5612ef70a6d09e22f83f8117cdbf2a0a053b8
Fix: 260769010