Cleanup DisplayTopologyPreferenceTest

- Added test display id and density constants
- Modified test to use DEFAULT_DISPLAY as the main displayId in
test as mirroring mode assumes DEFAULT_DISPLAY to always be present.
This doesn't affect non-mirroring mode tests
- Fixed threeDisplaysMirroringGenerateBlocks test to include the
third display
- Group util functions together
- Cleanup other miscellaneous warnings

Bug: 401059862
Test: atest DisplayTopologyPreferenceTest
Flag: EXEMPT test cleanup
Change-Id: Ieac43b148e2145f80be4dff5c9a235949be3c9b3