Update Gaia API documents for #a50c2e2dd6d6cdfcec8f1a26b91eed4ca07989af
    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
- update gaia-ui-tests