UserVisibilityMediator: allow visible bg users on default display.

So far only the current user is visible in the default display and
other users could only be started visible on secondary displays, but
that restriction must be lift for automotive builds that only support
passengers (for example, when a 2nd instance of Android is run on the
back seat of a car).

This CL changes UserVisibilityMediator to allow that scenario,
although it's not hooked with other parts of the system yet (which
will require a new config and new CTS tests).

Test: atest UserVisibilityMediatorMUPANDTest \
            UserVisibilityMediatorMUMDTest \
	    UserVisibilityMediatorSUSDTest
Fixes: 261538337
Change-Id: Ia7989f5ce2af41c7be2b1d578ab0e0e2aecc4e9d