Cleanup displaylib tests

- Moves fakes from systemui to displaylib
- Makes DisplayRepositoryTest use kosmos for deps
- Adds docs to the DisplayRepositoryTest in display lib referring to the systemui test class
- Makes the choice to duplicate pairwiseBy deliberate

Bug: 362719719
Bug: 401305290
Test: DisplayRepositoryTest
Flag: NONE - Just a cleanup of tests
Change-Id: Idee9505199caa6175b2a583767fc1965f84824b5