audio: fix race when stopping a stream

Stopping a Legacy stream could result in the stream
becoming killed. Then it could not be restarted.

This change results in the stream callback just returning safely.

Bug: 192095564
Test: see bug report
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I2d58e94bf9e7288b56e750e03188102b383b6479