Fix the logic to check if extra display is already assigned.

Check whether a user is already assigned to the requested extra display.

Bug: 354837901
Flag: EXEMPT bugfix
Test: atest CtsAccessibilityServiceTestCases:AccessibilityDisplayProxyTest#testRegisterAccessibilityProxy_withNonDeviceDisplay_throwsSecurityException
Test: atest FrameworksMockingServicesTests:UserVisibilityMediatorSUSDTest
Test: atest FrameworksMockingServicesTests:UserVisibilityMediatorMUMDTest
Test: atest FrameworksMockingServicesTests:UserVisibilityMediatorMUPANDTest
(cherry picked from https://partner-android-review.googlesource.com/q/commit:0e4e4691615862d0c04c9a0d651b79025af54452)

Change-Id: Ia5c4713ed46e0823dc1aa44fb3032a3999191aa3