Add the contrast API to UiModeManager

Part of the move of the API from AccessibilityManager to UiModeManager.
The logic remains unchained. a new AIDL, IUiModeManagerCallback, was
created to let the service notify all UiModeManagers about contrast
changes, similarly to what was done in AccessibilityManager

Constants (min, max, default contrast) have been moved to a new static
class "ContrastUtils" inside "UiModeManager".

To avoid creating too many CLs, a few new hidden constants/utils have
been added to "ContrastUtils", even though this is not part of the move.

Test: atest UiModeManagerTest
Bug: 259091608

Change-Id: I0c1cf5accc8b301ea90b81cdc20d5262eef04a8b