Publish MR2 system media session APIs

Also, pass parameters in a dedicated Params class, to enable future
extensibility without adding overloads, which will presumably be needed
for video.

Bug: b/362507305
Test: atest CtsMediaBetterTogetherTestCases:android.media.router.cts.SystemMediaRoutingTest
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I0936feefb4c92fe46212ad6bd71ed2b544286cbd