[bc25] Create ACONFIG flag for the BC25 Dual Shade feature.

Test: It builds.
Bug: 337259436.
Fix: 337021582.
Flag: ACONFIG com.android.systemui.dual_shade DISABLED
Change-Id: Iced87334092be4df92bd7de8edeb307f0609b920
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index fadef1e..a51f81f 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -226,6 +226,13 @@
 }
 
 flag {
+    name: "dual_shade"
+    namespace: "systemui"
+    description: "Enables the BC25 Dual Shade (go/bc25-dual-shade-design)."
+    bug: "337259436"
+}
+
+flag {
     name: "keyguard_bottom_area_refactor"
     namespace: "systemui"
     description: "Bottom area of keyguard refactor move into KeyguardRootView. Includes "