audio: Add compatibility bits to use legacy libaudio implementations

Enable with BOARD_USES_AUDIO_LEGACY, requires the use of the generic
legacy audio shim

Change-Id: I60407b48c2a8f3c9736805f31dcdef280c9aa485

Conflicts:

	media/libmedia/AudioSystem.cpp

Conflicts:
	media/libmedia/AudioSystem.cpp