bug 823450 - Add data connection warning dialog r=alive, kaze

https://bugzilla.mozilla.org/show_bug.cgi?id=823450

The original patch has been reviewed by Kaze (https://github.com/mozilla-b2g/gaia/pull/7106) and alive (https://github.com/mozilla-b2g/gaia/pull/7749). Create a new pull request here for requesting to merge to the master branch.

A few modifications have been made to address their comments:
- Use a div to show the warning dialog instead of an attention screen.
- Use async storage for recording if the warning dialog has been shown before.
- Remove the the CSS styles overriding the default confirm dialog style.