WiredAccessoryObserver: Headset detection for sony msm7x27a

Sony's headset driver uses the nonstandard headset_sensor switch
while other drivers (eg power button driver) still use the h2w
switch, so simply renaming back to h2w is not simple. This looks
for the headset_sensor switch, and if it's not present falls
back to the standard h2w switch.

Change-Id: Ibfd39edf80e2e3a2f4c084960908d1e24808df54