Add permission RAW_AUDIO to obtain the "audio" gid.

An app requesting this permission like this:
<uses-permission android:name="android.permission.RAW_AUDIO"/>
obtains the "audio" gid (group id), giving it read/write access
to files such as "/dev/msm_pcm_out".

This can be used, for example, for low-latency audio which is not
otherwise possible.

Change-Id: I745adc5996dd174d1cee737f305c5242097a9154