Add SYSTEM_APPLICATION_OVERLAY

.. which allows the holder to create privileged application overlays
that won't be hidden by Window#hideOverlayWindows

Bug: 159616727
Bug: 171177581
Test: atest CtsWindowManagerDeviceTestCases:HideOverlayWindowsTest
Change-Id: I1a8c08047ca51b12067fa68bea61843013940643