New UserManagerInternal APIs for extra user / display assignment.

Currently, a user is only visible in one display, which is assigned
when the user is started. But there will be cases where the user
should be visible on multiple displays, like on cars where the
back passenger seats have 2 displays each (one "main" display and
a smaller display for user input)

Test: atest UserVisibilityMediatorSUSDTest UserVisibilityMediatorMUMDTest
Bug: 261917465

Change-Id: I824f9663ef83451239a94f656eadba138a6ae50f