Bug 948847 - [Gaia] add AirplaneModeHelper

- other apps can use AirplaneModeHelper to know current airplane status
- add a lot of related unit tests
- add two mozSettings key for airplane mode use
Merge pull request #14836 from EragonJ/bug-948847

Bug 948847 - [Gaia] add AirplaneModeHelper to make sure settings app and system app will not do the same thing