Play button flashes when playback starts or ends

[Boris Burtin]

The play button flashes when the audio clip starts or stops playing. This happens because there's a short delay between the time that the user releases the button and when Player sends the intent that updates the button state. The UI code should change the state optimistically, and then only revert to the default state if something goes wrong during playback.

This is also related to issue #12. We need to make sure that the button state is intuitively updated during fade out and when the user cancels the fade.