Mute phone when flipped over while ringing.

When a phone is ringing, flipping it over will silence the ringtone for
that call. This is especially useful when you're in a
meeting/class/other gathering where lots of noise is frowned upon as it
allows for an extra fast silence.

Also has option to hang up the call. Configurable.

Change-Id: If7c2e64e002adbf5a3c8a16c6e6e7b7b6a86fbce