Responsiveness improvement for BrightnessObserver

Forget the history to get immediate response when entering lower
brightness environment.

The sensor may not report event if there is no change since last report.
BrightnessObserver should keep refreshing the filter's estimation when last
sensor value is in a higher zone.

Bug: 138404221
Exempt-From-Owner-Approval: Santos is designated by Michael
Change-Id: I4ab37c067826c36eda26b435e83258d9af8783ab