Adjust NSSL tests when footer flag is enabled.

Some tests in the NSSL were failing if the footer refactor flag was
enabled. Added a @DisableFlags annotation for all the tests where the
behaviour is already implemented (and tested) in the refactored stack.
All of these tests have an equivalent inside FooterViewModelTest or
NotificationListViewModelTest.

The footer visibility is not yet refactored, so those tests should
remain for now, but I adjusted them a bit so they work with the flag
both on and off (added atLeastOnce() since when the flag is on there's
an extra call to updateFooter). These will eventually have DisableFlags
as well, and later be deleted.

A few more tests complained about the
enable_notifications_simulate_slow_measure classic flag not being set,
so I fixed that as well.

Bug: 293167744
Test: ran the tests with the flag on to make sure they pass
Flag: N/A
Change-Id: I21e67ea9a1b27c8e18d52f9dfc6e010f2f29f819