Refactored UserVisibilityMediator unit tests:

Currently, there's just one "mode" for multiple users on multiple
displays, but soon there will be a new one, so this CL creates a
superclass to make it easier to test scenarios that are common for
both.

It also:
- Renames some tests to make it clearer whether the background
  user / profile is being started visible or in the conventional way.
- Fixes some tests that were asserting the wrong user ids.
- Adds extra tests for the current user visibility.

Test: atest UserVisibilityMediatorSUSDTest UserVisibilityMediatorMUMDTest
Bug: 261538337

Change-Id: I366ec38c73b3401d1a2913eb37a4fcc9668d5287