Tidy Up BrightnessEvent

It looks like this now:
DisplayPowerController[0]: BrightnessEvent: brt=0.051387966 (64.0%),
nits= 93.82994, lux=202.59569, reason=automatic, strat=AutomaticBrightnessStrategy,
state=ON, policy=BRIGHT, flags=, initBrt=0.049399007, rcmdBrt=0.051387966,
preBrt=0.049399007, preLux=337.86658, wasShortTermModelActive=false, autoBrightness=true,
autoBrightnessMode=default, unclampedBrt=0.051387966, hbmMax=0.33444074, hbmMode=off,
thrmMax=1.0, rbcStrength=35, powerFactor=1.0,
physDisp=Built-in Screen(local:4619827257460818176), logicalId=0

- additionally, remove references to dpc2
- clean up unused variables in displaypowercontroller

Bug: 353657111
Test: atest BrightnessEventTest
Flag: TEST_ONLY
Change-Id: I54febe053c5a3026f80ddb43506256f34c674716