Auto GPS: no GPS dialog

FB129325: Auto-gps forms crash if location isn't enabled.

This is pretty awful, but Android is *really* not designed to let you pop up a dialog outside the context of an activity. Tried putting the no-GPS dialog in its own activity, but wasn't able to launch it from PollSensor. I'm open to other suggestions.

There's a corresponding PR in commcare-odk because sticky broadcasts require a permission.

And no, I don't know why Will's commit is showing up in this PR.
Does this need to get unregistered when the activity ends? That was an issue for the homescreen, right?