libaudio: Samsung external amplifier (extamp) support

Driver checks for property persist.sys.extamp, which should
contain five comma-separated values, corresponding to the following:

enable (0,1)
headset (all except no-mic & fm)
no-mic headset
speaker
fm (headset/speaker)

This commit also enables extamp support in GalaxyParts application
(to manage this property).

Change-Id: I7015e3c5e0d4a6a94a08a071e5863ac95fd1313d