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: I641634a76ab4fc3cb4bf26134a96581329dbaf3f