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
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 780a099..09c3614 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -800,6 +800,13 @@
 }
 
 flag {
+    name: "brightness_slider_focus_state"
+    namespace: "systemui"
+    description: "enables new focus outline for the brightness slider when focused on with physical keyboard"
+    bug: "329244723"
+}
+
+flag {
    name: "edgeback_gesture_handler_get_running_tasks_background"
     namespace: "systemui"
     description: "Decide whether to get the running tasks from activity manager in EdgebackGestureHandler"