Fix flaky AudioFocusTest#testAudioMediaFocusDelayedByCall

When using external focus policy, the caller thread
requesting audio focus for test could be blocked while the
dispatch callback is prioritized. This results in the
client id not being added in time for the dispatch call
resulting in null pointer reference and finally request fail.

Fix it by ensuring the client id is added before handling
the response.

Bug: 303223553
Bug: 302870089
Test: atest -c --rerun-until-failure 1000
         AudioFocusTest#testAudioMediaFocusDelayedByCall
Change-Id: I7b2fe8601ca2b502501ee6af64f78d97a5be1f20