Support new Autofill session after service returns a null FillResponse.

When the AutofillService returns a null FillResponse for onFillRequest(),
AutofillManager was ignoring any further view calls backs (other than a manual
request to start a session) to optimize performance by avoiding unnecessary
IPCs to system_server (and then from system_server to the Autofill Service
process).

But this optimization has a drawback: it makes it harder for the service to
handle cases where the activitity dynamically added new views after a view
has been focused.

This CL offers a compromise to fix both problems: it removes the optimization
for notifyViewEntered() calls (as that's what triggers autofill), but still
ignores the other calls once the session is on finished state.

Test: atest CtsAutoFillServiceTestCases:LoginActivityTest#testAutofillAutomaticallyAfterServiceReturnedNoDatasets
Test: atest CtsAutoFillServiceTestCases:LoginActivityTest#testAutofillAutomaticallyAndSaveAfterServiceReturnedNoDatasets
Test: atest CtsAutoFillServiceTestCases:LoginActivityTest

Fixes: 70046972

For optimization purposes, AutofillManager was ignoring all request
Changed AutofillManager to start a new session after service returns a null

Change-Id: Iaae9ed7f8b12ebc9da13e990ef468d9019d5c6ca