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
Change-Id: I50c837a57d7aabf255685935539914723a38cb75