Don't call setAutofillClient() on base Context if it's null.

Test: atest CtsAutoFillServiceTestCases
Test: changed activity on autofill sample to override attachBaseContext() by
      calling super.attachBaseContext(null) - the activity still crashes, but
      not on attachBaseContext().

Fixes: 79707861

Change-Id: Idfaa9c596cc338ab815441138ce06ba64cd3b4d2

core/java/android/app/Activity.java

@@ -979,7 +979,9 @@ public class Activity extends ContextThemeWrapper
     @Override
     protected void attachBaseContext(Context newBase) {
         super.attachBaseContext(newBase);
-        newBase.setAutofillClient(this);
+        if (newBase != null) {
+            newBase.setAutofillClient(this);
+        }
     }
 
     /** @hide */