bug 942751 - stop OscillatorNodes when done to work around bug 897796

This patch also reverts to the behavior of the prior mozWriteAudio
implementation where stop() only stops long presses, allowing short presses to
release appropriately.
Merge pull request #14525 from karlt/942751

bug 942751 - stop OscillatorNodes when done to work around bug 897796 r=etienne