Make Extra Dim Setting as percentage of device

- Extra dim setting now operates in percentage of the device's
  capabilities
- ie "100" now means the maximum possible dimming on the current device,
  if restored to another device, this means the max possible dimming on
  that device as well.

Bug: 337351445
Flag: EXEMPT bugfix
Test: atest DisplayServiceTests
Test: atest ReduceBrightColorsIntensityPreferenceControllerTest
Change-Id: I98d0d7af562fdfb2b921746e0e5654bad47b440b