The Java PeerConnection maintains a cached list of Java RtpSenders
and RtpReceivers so that the same objects are returned every time
getSenders() or getReceivers() is called. They are disposed of when
PeerConnection.dispose() is called, which will also dispose their
referenced MediaStreamTracks.
Review URL: https://codereview.webrtc.org/1368143002
Cr-Commit-Position: refs/heads/master@{#10189}
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