Add focus outline to the brightness slider

This CL consists of two parts:
1. Showing the focus background of ToggleSeekBar
2. In order for the background to be correctly rendered, we need to
expand the boundary of brightness slider (BrightnessSliderView uses
hardware layer so it crops its children)

Bug: 329244723
Flag: ACONFIG com.android.systemui.brightness_slider_focus_state DISABLED
Test: SwipeBrightness.kt
Change-Id: Ia8d88ac27c20a5354254937a7cf0e2d03f049ac6