VolumePreference: fix audio sample playback

Handle audio sample start/stop request in a handler
to avoid race conditions.

If a sample is active, do not ignore playback requests but
schedule a second attempt one second later. This is to avoid
ignoring a playback request if a sample is short or about to finish.

Bug: 8176776.

Change-Id: If2d5f7d469eba345af3f4771067a076a56f4bfed