on
Call this method when playback of the utterance was stopped by something other than this synthesizer — for example the underlying speech engine being taken over or shut down.
Do not call it when TtsSynthesizer.pause or TtsSynthesizer.clearAndPause stopped playback: those were requested by the SyncMediaPlayer, which already knows playback stopped. Finishing one utterance and moving on to the next is not an interruption either — report that with onUtteranceEnd.