Add flag for hiding App Handle on immersive mode changes

To hide/show the app handle whenever the app requests immersive mode
while the app is in fullscreen.

Bug: 336368019
Test: m
Change-Id: I165685b40ffbc7651166655c51f976f4bf7e5209

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

@@ -57,3 +57,10 @@ flag {
     description: "Enables desktop wallpaper activity to show wallpaper in the desktop mode"
     bug: "309014605"
 }
+
+flag {
+    name: "enable_desktop_windowing_immersive_handle_hiding"
+    namespace: "lse_desktop_experience"
+    description: "Hides the App Handle when in fullscreen immersive mode"
+    bug: "336368019"
+}