Add support for system-wide mirroring

This change also disables per-app mirroring, because it's not really
supported by the rest of the android system:
- Audio focus prevents two apps from playing at the same time.
- Volume key routing needs substantial changes.
- Volume sliders don't support per-app routing either.

With this change, we can focus on implementing mirroring for the MEDIA
stream across the entire system, and on top of that, start iterating
over towards per-app routing.

Bug: b/362507305
Test: atest CtsMediaBetterTogetherTestCases:android.media.router.cts.SystemMediaRoutingTest
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I18f86f9f6246498b1b7007efdfafa286e645d7c2