BTAudio: report session ended after cleaning up

While Bluetooth stack ending a session, this CL does clean up first, and
then reports the change. All observers are able to get the correct
session state by invoking IsSessionReady() within the change callback.

Bug: 162065063
Test: manually
Change-Id: I920ea578d626d2e5844eb7c8a701636e20d7a219
1 file changed