Fix crash in TetherSettings.

In onCreate(), it checkes whether ui is restricted, and try to show an
empty screen by creating a new preference screen. However, when the
preference is not attached, it failed to get the next preference id.
Instead of replacing the current screen with a new one, remove all the
preferences on the screen.

Change-Id: I085bee025cfeabaf4aefacb717d145e668bd85e1
Fix: 36589767
Test: manual

src/com/android/settings/TetherSettings.java

@@ -142,7 +142,7 @@ public class TetherSettings extends RestrictedSettingsFragment
         setIfOnlyAvailableForAdmins(true);
         if (isUiRestricted()) {
             mUnavailable = true;
-            setPreferenceScreen(new PreferenceScreen(getPrefContext(), null));
+            getPreferenceScreen().removeAll();
             return;
         }