Add DisplayBrightnessEventOccurred atom.

This will be first used to log manual slider events. The plan is to
extend it to logging non-manual brightness events as well. For that more
reasons will be added to the enum Reason of the atom.

Test: statsd_testdrive
Bug:236234158

Cherrypick two additional changes: I08c03b248f4ee295a104759a523acbade239f1de, I2ec2496322c320e8acb7c0e15542851ae6533898,
I7213df42940dd1fd829a498dae00233f30cf3e6e

Change-Id: Ifa023d02bb35f95a66f3c94d9477f8ce59326218
(cherry picked from commit 46428cda2704622637c363429c4422b30deafda4)
Merged-in: Ifa023d02bb35f95a66f3c94d9477f8ce59326218