Add Unique Identifier for MediaSessionRecord(s)

- This is part of the work done for b/295518668 and b/297052684 we need
to differentiate between MediaSessionRecords owned by the same app.

- For example an app should be allowed to go to the foreground if it
has at least an active media session in a user-engaged state.

- In case an app has 2 active media sessions that are in a user-engaged
states, moving one of them to user-not-engaged state shouldn't prevent
the app from being in the foreground because of the other session.

Bug: 322776229
Test: adb shell dumpsys media_session
Change-Id: I0958d9b955468757a94eb82c53fde29eb243c13b