Move start/stopClient to command queue.

If the two methods is called from external to a service stream, it
should be protected by the lock and executed from the command thread.
The reason is that the two methods will access the endpoint, it will
cause race condition if it is not protected by the lock.

Bug: 341627085
Test: atest AAudioTests
Test: OboeTester disconnect test
Flag: com.android.media.aaudio.start_stop_client_from_command_thread
Change-Id: I8708e19332bc89389158fe3eda5b99f814e4fa22