Bug 1051803 - Add an optional vibration to the incoming call tone

I think that we are going to call `Settings.reset` more than once during the first run (take a look to https://github.com/mozilla-b2g/firefoxos-loop-client/pull/137/files#diff-abb510c96091b0d61e7b3e62b4199de0R179). Could we check this? Probably avoid a reset if there is other reset running?