MediaProjectionManagerService: Exempt privileged apps from the f/g service requirement.

Privileged apps targeting Q will not require a foreground service in
order to obtain a media projection permission grant. They will still
need to request permission for every projection session and will be
subject to a statusbar notification for the duration of the session.

Test: atest SurfaceControlTest AudioPlaybackCaptureTest
Test: (manual) MediaProjection with a priv-app that targets Q.
Change-Id: Ic88a7ec601a9f8e9a94759b0f7a9ec40034e66f3