Rename getDisplayAssignedToUser() to getMainDisplayAssignedToUser().

Now we have getDisplaysAssignedToUser() methods that returns all
displays assigned to a user, we are renaming the existing
getDisplayAssignedToUser() method to getMainDisplayAssignedToUser() to
make it more clear that it only returns a main display, and to avoid
potential confusion.

Bug: 272366483

Test: atest UserVisibilityMediatorSUSDTest \
            UserVisibilityMediatorMUMDTest \
            UserVisibilityMediatorMUPANDTest \
            UserVisibilityTest

Change-Id: Ia6dd667774c9eedd06c5dbb42575ca865b610994