Make MediaSessions listen for system media session overrides

This enables the sys ui volume slider to show up for volume adjustment
events in mirroring sessions. We still need to change routing session
updates to only show the slider as a result of a hardware volume key
press.

Bug: b/396394220
Test: atest CtsMediaRouterTestCases
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I1fccb4cf3891a1c61c92ea3dad9e97f7b7fc27ce