Partial fix for SoundPool not terminating

SoundPool was waiting for EVENT_UNDERRUN only to indicate end of clip.  In
J, AudioTrack delivered both EVENT_UNDERRUN followed by EVENT_BUFFER_END.
However, as of K, AudioTrack is only delivering EVENT_BUFFER_END (this
lack of EVENT_UNDERRUN is another bug which still needs to be fixed).
The workaround is to also respond to EVENT_BUFFER_END in SoundPool.

Bug: 10787103
Change-Id: Id68a23bddd6dd9df6c49c55138197260d71ca468