Add support for system media session volume adjustment

Also change the separate to be an underscore. The dot is already used
for the package name, which is part of the generated original id.

Bug: b/362507305
Test: atest CtsMediaBetterTogetherTestCases:android.media.router.cts.SystemMediaRoutingTest
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I6b078b80bfc970e8b9e4c0822cc460fccd45659d