Fix RtpReceiver use after free on Android

[alexlapa]

Synopsis

Shutting down VM
FATAL EXCEPTION: main
Process: com.instrumentisto.medea_jason_example, PID: 2160
java.lang.IllegalStateException: RtpReceiver has been disposed.
  at org.webrtc.RtpReceiver.checkRtpReceiverExists(RtpReceiver.java:86)
  at org.webrtc.RtpReceiver.id(RtpReceiver.java:48)
  at com.instrumentisto.medea_flutter_webrtc.proxy.PeerObserver.onTrack$lambda-5(PeerObserver.kt:72)
  at com.instrumentisto.medea_flutter_webrtc.proxy.PeerObserver.$r8$lambda$WDlx3s_cMBVUMU8Z78fPRNKjgGY(Unknown Source:0)
  at com.instrumentisto.medea_flutter_webrtc.proxy.PeerObserver$$ExternalSyntheticLambda4.run(Unknown Source:4)
  at android.os.Handler.handleCallback(Handler.java:938)
  at android.os.Handler.dispatchMessage(Handler.java:99)
  at android.os.Looper.loopOnce(Looper.java:201)
  at android.os.Looper.loop(Looper.java:288)
  at android.app.ActivityThread.main(ActivityThread.java:7839)
  at java.lang.reflect.Method.invoke(Native Method)
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)

Checklist

• Created PR:
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[alexlapa]

FCM:

Fix `RtpReceiver` use after free on Android platform (#165)