Fix iOS background double-audio: pause the media-element route on handoff

After the background-WAV handoff landed, backgrounding on mobile played BOTH the WAV (good — music continues) AND a looping chunk. The chunk was the media- element route's : it plays the live MediaStream, and when iOS suspends the ctx it loops that stream's last buffer.

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f949858 https://github.com/aboard-io/stellate/commit/f94985867226579bd1eb0f6186c70c18124fdb7f