Change DisplayModeDirector's brightness source to DisplayListener

Currently, DisplayModeDirector listens to brightness changes from Settings.
However, brightness value in Settings is overridden for various reasons
inside DisplayPowerController before being propagated to HALs. As a
result, the brightness value that DisplayModeDirector is acting on isn't
always the brightness value that gets applied to the display.

This change,
- Changes DisplayPowerController so that the final brightness value (after all
modifications) is saved.
- Changes DisplayModeDirector to use DisplayManager.DisplayListener to
observe brightness changes and then retrieve and act on the final
brightness value saved in DisplayPowerController.

Bug: 199955532
Test: atest DisplayModeDirectorTest
Change-Id: Ia6dd7d194b6913d2942bea9c29a2011b8058beef