sys user split: first pass updating Settings

Update various places in Settings to use "admin" ueser flag
instead of checking user id "0".  This should be no-op in
single user mode since the only admin user would be user 0.
In split system user mode, this will correctly ACL admin
user instead of non-interactive system user.

Bug: 19913735
Change-Id: Ida4d59c5f689ea0dc34b3b3ff0822b087fa0afd6

src/com/android/settings/TetherSettings.java

@@ -125,7 +125,7 @@ public class TetherSettings extends SettingsPreferenceFragment
         mUm = (UserManager) getSystemService(Context.USER_SERVICE);
 
         if (mUm.hasUserRestriction(UserManager.DISALLOW_CONFIG_TETHERING)
-                || UserHandle.myUserId() != UserHandle.USER_OWNER) {
+                || !mUm.isAdminUser()) {
             mUnavailable = true;
             setPreferenceScreen(new PreferenceScreen(getActivity(), null));
             return;