Fix the issue of accessing BrightnessSetting inside a lock

Given BrightnessSetting is operating on the display lock, acquiring
power lock while accessing the BrightnessSetting can result in a
deadlock.

Bug: 278692011
Test: Manually verified the behaviour is intact
Test: atest DisplayBrightnessControllerTest
Change-Id: I40c8340895c165da3b96015bc3e8918c259876ac