Add API for virtual view is ready to user

When the view is ready to the user, notify autofill can do something
if needed before the user start to input. And then autofill start to
prepare suggestions for some features, e.g. a prefill request for the
fill dialog.

Bug: 258077433
Test: atest android.autofillservice.cts.dialog
Change-Id: I0c6c964fb158c2da35b3470528d92d0d5bf74aa7