Put taskbar all apps in separate overlay window and stash taskbar.

All apps should display below system UI components such as the
notification tray and power menu, so an overlay window is more
appropriate. As a result, all apps has a separate window activity
context, but some properties are delegated to the taskbar activity
context. Taskbar should also be stashed while all apps is open.

Change-Id: I593457708779d84a0ab8b949a966d247d0a2e1b7
Test: Manual
Bug: 216843189
Fix: 217383817