CoreAudio: Create an audio plugin supporting iOS and OS X

This removes the Mac audio backend that was hardcoded into QtMultimedia
and adds a new audio plugin using the CoreAudio API.

Change-Id: Ib15291825f9452a3763e0eeb281d952deb0bad3d
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Android: fix compilation on x86.

Task-number: QTBUG-33554
Change-Id: I4da7410d3b77ec6d72a73d3b6ceb7b76bb48d2b7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>