Enable credential suggestions without autofill provider

When the user does not select an autofill provider as the main password
provider, the autofill feature gets disabled. However, for views that
have credman requests, we still want to support providing credman
suggestions. To enable this, credential autofill service will be the
default autofill provider if there is no valid autofill provider. The
autofill session, in this case, will only be activated when the views
have credman requests.

Bug: 330177328
Test: cts
Change-Id: I9a7c8c6763d2814ed5a46790442f0133f0bd7537