Make sure user tracking stops

There was one case of the user tracking not stopping, which resulted
in the incorrect number of notifications showing. Also simplify the
notification count logic slightly.

Test: atest SharedNotificationContainerViewModelTest
Bug: 296606746
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl DEVELOPMENT
Change-Id: I8a572087ba8c44d80cda192567fa187c4e132464