SF: Use display device factory in unit test

This refactors the unit test to use the new factory for display device
instances.

Test: libsurfaceflinger_unittest passes on Pixel XL
Bug: 74827900
Change-Id: Icf7cf2fda49521de3f186190e151eb2eb5f72496
(cherry picked from commit 0b1fe70e405e28a5d5cd402935bdff064ff9e541)
2 files changed