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

src/com/android/settings/Settings.java

@@ -98,5 +98,6 @@ public class Settings extends SettingsActivity {
     public static class QuickLaunchSettingsActivity extends SettingsActivity { /* empty */ }
 
     public static class TopLevelSettings extends SettingsActivity { /* empty */ }
+    public static class ApnSettingsActivity extends SettingsActivity { /* empty */ }
 }