Add missing test coverage

In preparation for a minor refactor. Rather than writing
tests for the code guarded by ENABLE_LOCK_SCREEN_ALLOW_REMOTE_INPUT,
I've deleted it, since it's been disabled since at least 2017.

Test: NotificationLockscreenUserManagerTest
Bug: 286230167
Change-Id: Id31bd0f997022ac7b4ad527120e45e47736def12