Polish transition between desktop mode and overview

Fixing desktop task in place along the x-axis and animating its background's alpha to more smoothly handle the transition.

Flag: com.android.launcher3.enable_desktop_task_alpha_animation
Fixes: 320307666
Test: swiped to and from desktop mode and overview
Change-Id: I38d62d0c966733f96c2f4d4ecfa4a9418039fd48
diff --git a/aconfig/launcher.aconfig b/aconfig/launcher.aconfig
index be8ebbb..dbcf49c 100644
--- a/aconfig/launcher.aconfig
+++ b/aconfig/launcher.aconfig
@@ -339,3 +339,13 @@
     description: "Change fast scroller to a lettered list"
     bug: "358673724"
 }
+
+flag {
+    name: "enable_desktop_task_alpha_animation"
+    namespace: "launcher"
+    description: "Enables the animation of the desktop task's background view"
+    bug: "320307666"
+    metadata {
+      purpose: PURPOSE_BUGFIX
+    }
+}