Fix bug #16703191 APN settings shows back button even when you intent into it

- change a bit the way we recognize a SubSetting. Now we can pass the new
":settings:show_fragment_as_subsetting" Intent extra for forcing it.
- convert ApnSettings to a SettingsPreferenceFragment

Change-Id: I22167ad317530c0a58c4d522a72826f9d34ad2d3
6 files changed