Simplify MediaSessionRecord callback invocations

And add a comment about thread safety for the callback holders.

This change doesn't make user-impacting functional changes.

Bug: 205124386
Test: atest CtsMediaBetterTogetherTestCases CtsMediaHostTestCases
Change-Id: Ifbc97b3e7fdb958922a794afd2416dce62df9cc2
1 file changed