Temporary workaround for issue 4604090

There was a regression caused by a change in Honeycomb that removed
mediaserver process from system group to improve security: This prevents
the audio HAL (which runs in mediaserver process) from taking wake locks
while audio is playing thus causing the pcm stream to get unexpectedly
suspended.

This temporary workaround is just to allow ICS dogfood in better conditions until
a real fix taking into account the security issue is implemented.

Change-Id: Ic19a23da4b4383559188b6f5c3a82604c3d95d74