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
(cherry picked from commit 473b980bf9)