This also gives us the flexibility to release a stream while keeping the tracks around, which is necessary sometimes.

The implementation tries to be as faithful to the standard as possible,
however an extra function call is still required for cleanup:
MediaStream.release().

Now that a track can outlive the stream that originated it, there is
also MediaStreamTrack.release(). Generally it's not needed to call it
unless you have already released the stream and want to dispose of the
track alone.

Add / remove track operations on streams are only supported for local
tracks (ie, those gotten via getUserMedia).

Note: calling release() on a track still attached to a stream is
considered undefined behavior and may lead to crashes.

MediaStream's id is unique only within a PeerConnection. But
WebRTCModule presumed that it was globally unique. (Re)introduce a
reactTag in the JavaScript MediaStream to make sure that the Java
WebRTCModule is able to distinguish equal MediaStream ids based on the
Java MediaStream instance reference.

- import statements need to be determinated by semicolons too
- if we're using ES6 imports now, we might as well use ES6 exports too
- strive to stay within 80 chars per line

track setEnabled and media stream release track

It's not specific to the RTC spec, so it shouldn't have that prefix. The browser also calls it MediaStream, not RTCMediaStream. Lastly, implement spec as best as possible. See https://www.w3.org/TR/mediacapture-streams/#mediastream