Convert IMPROVED_HUN_ANIMATIONS flag to aconfig

Convert a classic flag to aconfig.

Bug: 25690710
Flag: ACONFIG notifications_improved_hun_animation DEVELOPMENT
Test: m droid
Change-Id: I2d07d86d29a6a1d38321f3f0cadb96b241d8caba
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index a26b311..933279a 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -45,6 +45,13 @@
 }
 
 flag {
+    name: "notifications_improved_hun_animation"
+    namespace: "systemui"
+    description: "Adds a translateY animation, and other improvements to match the motion specs of the HUN Intro + Outro animations."
+    bug: "243302608"
+}
+
+flag {
     name: "notification_lifetime_extension_refactor"
     namespace: "systemui"
     description: "Enables moving notification lifetime extension management from SystemUI to "