Make WifiTether listen to datasaver

Wifi Tethering is already turned off when appropriate. However
if you re-enable it you still can't toggle the tether switchbar.
This CL makes it so that if you disable data saver the toggle
becomes enabled again.

Test: robotests
Bug: 77860936
Change-Id: I03fa74c21674678a90cde7bd20fa68167f02ae3c

tests/robotests/src/com/android/settings/wifi/tether/WifiTetherSwitchBarControllerTest.java

@@ -88,4 +88,20 @@ public class WifiTetherSwitchBarControllerTest {
         assertThat(mSwitchBar.isChecked()).isFalse();
         assertThat(mSwitchBar.isEnabled()).isTrue();
     }
+
+    @Test
+    public void testOnDataSaverChanged_setsEnabledCorrectly() {
+        assertThat(mSwitchBar.isEnabled()).isTrue();
+
+        // try to turn data saver on
+        when(mNetworkPolicyManager.getRestrictBackground()).thenReturn(true);
+        mController.onDataSaverChanged(true);
+        assertThat(mSwitchBar.isEnabled()).isFalse();
+
+        // lets turn data saver off again
+        when(mNetworkPolicyManager.getRestrictBackground()).thenReturn(false);
+        mController.onDataSaverChanged(false);
+        assertThat(mSwitchBar.isEnabled()).isTrue();
+
+    }
 }