Add trunk stable flag for ActivityEmbedding animation customization.

Bug: 293658614
Test: Build
Change-Id: Ie753f3a11c751a07fa8262e73904a47f158d48df

core/java/android/window/flags/windowing_sdk.aconfig

@@ -130,4 +130,14 @@ flag {
     metadata {
         purpose: PURPOSE_BUGFIX
     }
+}
+
+flag {
+    namespace: "windowing_sdk"
+    name: "activity_embedding_animation_customization_flag"
+    description: "Whether the animation customization feature for AE is enabled"
+    bug: "293658614"
+    metadata {
+        purpose: PURPOSE_FEATURE
+    }
 }
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