Enhance audio playback behavior (overlapping audio)

[zytegalaxy]

Right now when there are two or more back to back notifications, their audio messages overlap and sounds weird.

I suggest we introduce a new parameter/attribute that indicates a new audio track/message should stop any currently playing audio or not (allow overlapping audio).

I can see overlapping audio being useful in certain cases such as playing a short chime when the device is talking. However speech playing over speech should be avoided.

cc: @sassanh