Mac native audio - temporarily back out recent changes to AudioSystemClipImpl.java. This re-introduces 2 minor bugs in exchange for working ring-back and ringing notifications on Mac.

Backs out the following commits to this file:
d9758b
05f148
5a10d4
18036b
Merge branch 'MacNativeAudio' of github.com:Metaswitch/libjitsi into MacNativeAudio