Adding some brightness logging.

Logs each time there's a change to the rules that control brightness.
Example: screen_off -> automatic -> manual -> manual [dimmed]

Adds a few lines to the log on startup and during major transitions (screen
off/doze/on), but stays pretty quiet out of transitions.

This should help debugging some temporary brightness bugs that we get on
occasion.

Test: manual
Bug: 131412806
Change-Id: I4db53547b2edb25863cebc6d5f44f4dacc80a1a0