Implement baseline system media session management

This change enables the selection of system media routes for any app,
and implements the most important bits of session management, but
doesn't cover everything (for example, this change doesn't cover stream
expansion for system media routing sessions, or routing of the entire
system).

Bug: b/374321232
Test: atest CtsMediaBetterTogetherTestCases CtsMediaHostTestCasts
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: Id43c4568350db068f929d5c6463a079ee28a3471