Add UserManagerInternal#getDisplaysAssignedToUser() method.

The existing UserManagerInternal#getDisplayAssignedToUser() only returns
a main display that is assigned to a given user.  We need a method to
get all displays that are assigned to a user, including extra displays.

Bug: 270722344

Test: atest UserVisibilityMediatorSUSDTest \
            UserVisibilityMediatorMUMDTest \
	    UserVisibilityMediatorMUPANDTest

Change-Id: Ib67d2b6b8e58f1de08c0dfbdaee5e1a6176cefc8