Initial fix on Autofill for visible background users.

It needs to set the proper display in the FillUI context, otherwise
WindowManager doesn't allow the AutofillPopupDialog to be anchored to
the activity being autofilled.

Test: atest --user-type secondary_user_on_secondary_display CtsAutoFillServiceTestCases:android.autofillservice.cts.dropdown.LoginActivityTest#testAutofill_oneDataset
Test: atest CtsAutoFillServiceTestCases # on phone
Bug: 271031908

Change-Id: Ifa9a69101168888afccfc9f63aa8a4eb92a93108