Avoid multiple calls to start/stop FGS for the same session record

Shouldn't be an issue, as they should both be idempotent operations, but
following the principle of robustness, this CL avoids calling
startingFGSDelegate (or stop) multiple times in a row for the same
session.

Bug: b/295518668
Test: atest CtsMediaBetterTogetherTestCases
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsDelegateTest.java
Flag: com.android.media.flags.enable_notifying_activity_manager_with_media_session_status_change
Change-Id: Ie4a4d2e3b49329f5c4480f1ca4c2531b86fe39a2