Load even dimmer config mappings

- Allow minimum nits to be specified in even dimmer config
- Adjust strength based on requested nits
- Add tests

Bug: 369217612
Flag: com.android.server.display.feature.flags.even_dimmer
Test: atest DisplayServiceTests
Change-Id: Idcdaf087f13e74f7f61b1ad510db6d1eb9b17d67