API to show autofill dialog.

This allows a UI Toolkit to display the autofill dialog instead of an
input method, if autofill suggestions are already available when the
user focuses on a view.

The dialog typically covers the input method, so it is undesirable to
show the input method under it.

Bug: 210926084
Test: atest android.autofillservice.cts.dialog.DialogTest
Change-Id: Ib982b0fcc271ed3fa2bf43ebe2f9baf494ec82ef