Remove unnecessary check in AutomaticBrightnessController

We already return early if the brightness is within threshold.

If manually set, we should respect the user setting, even if it's a very small change.

Bug: 366329379
Flag: EXEMPT bugfix
Test: AutomaticBrightnessControllerTest
Change-Id: I1593f092163dc57c52994a8b05361b6805ba8fba