Follow up to ambient brightness stats change

- Add docs to public methods of the API
- Use single variable for scheduling write from brightness tracker

Test: atest com.android.server.display.AmbientBrightnessStatsTrackerTest
&& atest android.hardware.display.AmbientBrightnessDayStatsTest
Bug: 69406079
Change-Id: I828e3bde4691240a2c0e0443e60b30665b65b8de