Allow null fill values to support authentication case

If a dataset needs to be authenticated the fill service
may not have the values but needs to tell the system for
which fields to show the fill UI. We now allow passing
a null value to mean the view is a part of the dataset
semantically but its value should remain unchanged.

If a dataset has no values, i.e. the related autofill ids
are mapped to null, we cannot properly filter. In this case
we always match such items regardless what the user typed.

While at this improved accessibility support for filtering
to announce when the number of items being filtered changes.

Also while at this allowed a dataset authentication to return
a response which replaces the current response and refreshes
the UI. Matching datasets with null values to any text plus
allowing a response to be returned from a dataset auth enables
the use case where there is always "Import" item at the
end of the list which when clicked can show arbitrarily more
data entries associated to other apps.

Another change is that we now provide the client state
bundle on both request and dataset auth.

Finally, this change gets rid of dataset waiting auth and
response waiting auth concepts since the reference to the
response and the dataset is piped with the auth request.

Fixed a bug where the width of the autofill UI was not
properly measured by going over all items in the adapter.
Now we measure enough height to fit the first three and the
width id the width of the widest item in the adapter.

Test: Added LoginActivityTest#testDatasetAuthTwoFieldsReplaceResponse
      Added LoginActivityTest#testDatasetAuthTwoFieldsNoValues
      Added LiginActivityTest#filterTextNullValuesAlwaysMatched
      All autofill CTS tests pass

bug:37724701
bug:37424539

Change-Id: Ic19e5d7cbdbb7d110c9e7da0ad60b540cbf1aecf