Fragmentize BluetoothSettings.

Add UserLeaveHintListener.java, so that the settings won't use
its implementation around Activity#onUserLeaveHint().

Also fragmentize bluetooth tethering screen.

Change-Id: Id06ae3161fbdb5854ddb7a257f464fb16ea9b089

src/com/android/settings/SettingsPreferenceFragment.java

@@ -175,6 +175,10 @@ public class SettingsPreferenceFragment extends PreferenceFragment
         return mNextButton;
     }
 
+    public void finish() {
+        getActivity().onBackPressed();
+    }
+
     /**
      * Sets up Button Bar possibly required in the Fragment. Probably available only in
      * phones.