Moves BrightnessUtils into SettingsLib DisplayUtils.

(Renamed from DisplayDensityUtils to be more generic)

This allows the AccessibilityMenu accessibility service APK
to include a much smaller dependency static library, decreasing
the post-optimization APK size from 3mb to 200kb.

Bug: 261252772
Test: build-only changes; check local & Treehugger builds
test: inspect AccessibilityMenu APK size
Change-Id: Iad36eb739ac38d923acabe558bc8f9c0395495d3