Update MultiDisplay_test for duplicate layerstacks

Duplicate layerstacks are no longer permitted, but existing
MultiDisplay tests do not catch these cases since the tests
do not go through the SF composite path where the duplicates
are rejected. Mirroring logic between physical and virtual
displays is no longer applicable, and an exisiting test already
captures this mirroring logic without duplicate layerstacks.

Bug: b/370358572
Test: atest MultiDisplayTest
Flag: EXEMPT, update test
Change-Id: I301c049740597865e72ea7450b692e5278dca24d