Route volume key presses to mirroring sessions while mirroring

This change doesn't implement the system ui changes, which will come in
a follow up CL.

Bug: b/362507305
Bug: b/396394220
Test: atest CtsMediaBetterTogetherTestCases:android.media.router.cts.SystemMediaRoutingTest
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: Icd9b1fa299093dded1ebcd52d31ada6d764be022