Fix crash if last tone in pack was disabled

Looks like a wrote a bad while loop in RingPackVm's
findNextEnabledTone(). Forgot to reset the search to the start of the
pack if we're at the end. Also avoids an infinite loop by returning
null.

The infinite loop case is if the user has NO tones enabled in a pack. We
won't let them set it in that case. NOTE: they could go back later and
disable all of them, which will crash us. I'm betting that won't happen.
Avoid uncommon crash

On the LG Optimus L9, somehow there was a valid ringtone URI, but it
wasn't in the MediaStore.
Add Edit>Toggle All action bar item

Useful for the bigger packs.
Pushed new version