Add RingerSwitch support

This patch adds support for the Geeksphone ONE's MUTE hard key.
I don't know of any other device with such a key, so this will
defaults to disabled. It's enabled by setting the "ro.config.ringerswitch"
property to 1

Change-Id: Ica18f3f3ee872b4b911ddd79b95ba569cb13f4e8