Bug 1024779 - [ringtones] edge gestures doesn't work

I think it's ok for other apps to handle the `manageringtones` activity (whatever we call it), since other apps can already handle the `pick` ringtone activity. It's only fair to let them manage their ringtones too.

In the long run, I'd be happy to see people be able to replace the ringtones app entirely, subject to some rules about how they can change the default ringer.