Add @TestApi to brightness @SystemApis

Expose the current @SystemApis as @TestApi
so they can be used from CTS.
Add missing equals to BrightnessCorrection
so that equals on BrightnessConfiguration works.

Test: atest android.display.cts
Bug: 122988010
Bug: 122954284

Change-Id: Ifdb79bd6bb8e3fcd3aa783aad09b08f81c73f433