Add API for creating AutofillId with virtual child id

App needs to create AutofillId for virtual views for client suggestion.
That requires new API otherwise client can not have any id with virtual
view id when request a response from client.

Bug: 261024705
Test: atest android.autofillservice.cts.unittests.AutofillIdTest
Change-Id: Ic78a548b414a47239c222192d2d29063341d6c35