audioflinger: Nasty hack for adjusting BCM FM volume

Normally this kind of nasty hack could go into libaudio, but on this
particular device, we have no access to the source code of libaudio.

Put the nasty hack into audioflinger and wrap it in ifdefs.

Change-Id: I5e3e495e3bd6b671823967b61ba5ceb49e59a401