Further improvements on isUserVisibleOnDisplay(userId, displayId).

Commit d4c41e18ca3b4767907ca6ca57ce8341cdae9f25 changed this method
behavior to always return true to the current user (and their
profiles); that should be the case on most cases, except when
the display is assigned to another user (i.e., a background user
that was started on that display).

This CL partially fixes that issue - it only returns true for the
current / user profile when a display is not assigned to another
user. It's not the ideal solution yet, because it would still fail
in 2 scenarios:

- It would allow the current user to be visible in a secondary
  display that is not being used by a passenger yet.
- It would not handle the case where the a virtual display should
  be visible only to the user that created it.

This CL also:

* Changed it to always return false for INVALID_DISPLAY
* Changed the is-user-visible command to allow INVALID_DISPLAY.
* Fixed startUserInternal() so assignUserToDisplay() is only called
  after checking if the user exists.

Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest UserManagerInternalTest
Test: adb shell am start-user --display 42 13 && \
      adb shell cmd user is-user-visible --display 42 0 &&
      adb shell cmd user is-user-visible --display -1 0

Bug: 244644281
Change-Id: I2803e0b5d49b8a729266e534bbe203e346720417