Fix issue 4604090: notification sound interrupted.

The problem is that the audio HAL fails to acquire the wake lock when playing the notification.
This is because of a change that removed the mediaserver process form the system group for honeycomb.

The fix consists in requesting the wake lock from PowerManagerService when AudioFlinger mixer
wakes up.

A consequence of this change is that audio HALs or pcm drivers do not have to hold wake locks
anymore as in the past.

Change-Id: I4fb3cc84816c9c408ab7fec75886baf801e1ecb5