Mark DataUsageState as nullable to avoid potential NPE risk

mDataUsageState is nullable in UnrestrictedDataAccessPreference.java, so we'd better
add a annotation to getDataUsageState(), and add a null-check before calling it.

Bug: 246828843

Test: manual and Robotest.
Change-Id: If7fea53ec7efd004391a09becdba5709ae686f40

src/com/android/settings/datausage/UnrestrictedDataAccessPreference.java

@@ -20,6 +20,7 @@ import android.graphics.drawable.Drawable;
 import android.os.UserHandle;
 import android.view.View;
 
+import androidx.annotation.Nullable;
 import androidx.preference.PreferenceViewHolder;
 
 import com.android.settings.R;
@@ -152,6 +153,7 @@ public class UnrestrictedDataAccessPreference extends AppSwitchPreference implem
         }
     }
 
+    @Nullable
     public AppStateDataUsageBridge.DataUsageState getDataUsageState() {
         return mDataUsageState;
     }