CoreAudio: make sure audio stops playing when the device is muted.

The audio session's category was set to Playback, which implies that
audio is still audible even after setting the device to silent mode or
locking the screen. This shouldn't be the default behavior.
We now set it to Ambient, which preserves mixing with other apps
but makes sure sound is turned off when it should.

Task-number: QTBUG-39036
Change-Id: Ic36668d73f3179dc38b41023e380e15f8c8517e0
Reviewed-by: Christian Stromme <christian.stromme@digia.com>