Display: Capture brightness adjustment events

Modified logBrightnessEvent to capture adjustments in brightness. This includes identifying if there were any increases or decreases between the initial and current brightness in nits.

Test: StatsD, Paramterized tests and manual (device).
Bug: 291013376
Flag: EXEMPT no API change
Change-Id: I1e20a0858ab53414c945deee85363dbe1fbb5701