feat: add first-packet observers, sender and receiver IDs, and track volume

[devopvoid]

Stacked on #335: the new observers end with its ReportPendingException helper. The base becomes main once #335 is merged.

More of the Android SDK's RtpReceiver, RtpSender and AudioTrack API.

First-packet observers

• RTCRtpReceiver.setObserver(RTCRtpReceiverObserver): onFirstPacketReceived(MediaType) when the receiver gets its first RTP packet, for example to show a remote video only once there is something to show.
• RTCRtpSender.setObserver(RTCRtpSenderObserver): onFirstPacketSent(MediaType) for the first packet a sender sends.

An observer set after the packet already went through is called right away, before setObserver returns, as WebRTC does. null removes it. Both run on the signaling thread, and an exception they throw goes to the thread's uncaught exception handler.

Lifetime: WebRTC requires an observer to be removed before it is destroyed. The native observer is held by the Java object it was set through (an observerHandle field, as RTCDataChannel does) and freed when it is replaced, set to null or that object is disposed, each time after SetObserver has handed it back. That call runs on the signaling thread, the thread that calls the observer, so no call is in progress once it returns. A receiver has one observer, so setting one through a second Java object of the same receiver replaces it, and disposing the first object then removes it; the javadoc says to set it through one object, such as the one onTrack provides.

IDs and streams

• RTCRtpReceiver.getId() and RTCRtpSender.getId().
• RTCRtpSender.getStreams(), the counterpart of setStreams(). A track added without streams has none, as Unified Plan does.

Remote track volume

AudioTrack.setVolume(double) sets the playout volume of a remote track, from 0 (silent) to 10, where 1 plays it as received. Values outside that range, and NaN, throw IllegalArgumentException: WebRTC only checks the range in debug builds. The gain is applied after the audio is handed to an AudioTrackSink, so sinks are unaffected, and a local track's source ignores it.

Docs

get-started.md gains a "Receiving Tracks" section with the receiver observer and the volume.

Tests

RTCRtpSenderReceiverTests:

• firstPacketEvents: in a real call with audio and video, the sender and both receivers report their first packet with the right media type, observers set afterwards are called during setObserver, and removing and disposing work.
• senderIdAndStreams, streamsOfSenderWithoutStreams, receiverId: IDs are the same through every Java object of a sender or receiver; stream IDs follow addTrack and setStreams.
• remoteTrackVolume, localTrackIgnoresVolume: valid volumes apply, invalid ones throw.

The full webrtc suite passes on Windows (259 run, 0 failures, 2 skipped), also under -Pjni-check with no JNI errors.