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