aaudio test: test whether close joins the callback

Check whether audio callback is still running after close().

Bug: 143305727
Bug: 141185837
Bug: 182954108
Test: Manual test. Enter:
Test: adb shell test_callback_race
Test: Watch to see if the test hangs.
Test: It will print PASS or FAIL if it does not hang.
Change-Id: I1c92259591cb221a8fe108c1b2a723c4f2930c57