Restore the pre-flexi code path of maxDisplayedNotifications

This CL is a cleanup to restore the old maxDisplayedNotifications API,
that was in place pre- I083472c936d9057544c71ab58a7162b15465f9ec.

Flexiglass needs to couple the values that are meant to be updated
together with maxDisplayedNotifications, so we've migrated this API to
provide a data class.
Later on the calculation of these values have diverted so much between
the legacy and the flexi path, that it didn't make sense to keep them
together, and flexi received its own LockscreenDisplayConfig from a new
interactor. This meant that we can clean up the flexi specific code from
the legacy maxDisplayedNotifications. That's happening in this CL.

Bug: 361324188
Test: Check the Notifications behaviour over LockScreen
Flag: EXEMPT refactor
Change-Id: I5b22e315c411b833fc4fe2bbd8dbb85cee747429