[Settings] Fix NPE from updating subinfo changed.

- Refactor code to avoid NPE.

Bug: 278721219
Test: make passed
Test: atest MobileDataPreferenceControllerTest passed
Change-Id: I1fe57b8835677335ae9102cfff71451060ea4f78

tests/unit/src/com/android/settings/network/telephony/MobileDataPreferenceControllerTest.java

@@ -20,7 +20,6 @@ import static com.android.settings.core.BasePreferenceController.AVAILABLE_UNSEA
 
 import static com.google.common.truth.Truth.assertThat;
 
-import static org.mockito.Matchers.anyInt;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.verify;