[Autofill] Migrate Flag

Convert aconfig flag to DeviceConfig flag so we can do an experiment in public beta.

bugfixes.aconfig#include_invisible_view_group_in_assist_structure

Bug: 327225585
Test: android.autofillservice.cts.dropdown.LoginActivityTest#testNoContainers

Change-Id: I32825fd2f37162622e1baed889b91bb724b3e05e