Moving to aconfig flag to control the haptic brightness slider plugin

Instances of the legacy feature flag were also removed. The feature flag
was also removed.

Test: atest SystemUITests:BrightnessSliderControllerTest
Bug: 296467915
Flag: ACONFIG com.android.systemui.haptic_brightness_slider DEVELOPMENT
Change-Id: I6b478a80aab933b661b46b57b509ee696882d092