Added postprocessor functionality to the strategy selector

With the plans to introduce AutomaticBrightnessStrategy, there are
requirements for strategies to be notified about the selection of a
DisplayBrightnessStrategy for a request.

Bug: 326947631
Test: atest DisplayBrightnessStrategySelectorTest
Change-Id: Ie4463de0aa577db94f653923677ac69ff227ad84
14 files changed