Check availability on start for TetherSettings

mUnavailable variable is only assigned when creating the activity. This make the app crash when we disable tethering and then resume + toggle on the Bluetooth Tethering. This cl remove the variable and directly check whenever the app start.

Bug: 270285631
Test: manual
Change-Id: I0dff1d11cbcc06816a69985f96eee828bb83e87c

src/com/android/settings/network/tether/TetherSettings.java

@@ -123,7 +123,6 @@ public class TetherSettings extends RestrictedSettingsFragment
     private boolean mMassStorageActive;
 
     private boolean mBluetoothEnableForTether;
-    private boolean mUnavailable;
 
     private DataSaverBackend mDataSaverBackend;
     private boolean mDataSaverEnabled;
@@ -168,7 +167,6 @@ public class TetherSettings extends RestrictedSettingsFragment
 
         setIfOnlyAvailableForAdmins(true);
         if (isUiRestricted()) {
-            mUnavailable = true;
             getPreferenceScreen().removeAll();
             return;
         }
@@ -360,7 +358,7 @@ public class TetherSettings extends RestrictedSettingsFragment
     public void onStart() {
         super.onStart();
 
-        if (mUnavailable) {
+        if (isUiRestricted()) {
             if (!isUiRestrictedByOnlyAdmin()) {
                 getEmptyTextView().setText(R.string.tethering_settings_not_available);
             }
@@ -388,7 +386,7 @@ public class TetherSettings extends RestrictedSettingsFragment
     public void onStop() {
         super.onStop();
 
-        if (mUnavailable) {
+        if (isUiRestricted()) {
             return;
         }
         getActivity().unregisterReceiver(mTetherChangeReceiver);