[Drag To Hide] Shows notification when user drag-to-hide a11y floating button

* Click on notification to bring back a11y floating button
* Swipe notification to remove a11y floating button and its services
* A11y service key changed to dismiss notification
* Align to create ACCESSIBILITY channel in systemui NotificationChannels

Bug: 298718415
Flag: ACONFIG com.android.systemui.floating_menu_drag_to_hide DEVELOPMENT
Test: atest MenuViewLayerTest MenuNotificationFactoryTest NotificationChannelsTest

Change-Id: Ie209ec946ea15903acfa6913492605277e9d7057
diff --git a/proto/src/system_messages.proto b/proto/src/system_messages.proto
index b403a7f..7f542d1 100644
--- a/proto/src/system_messages.proto
+++ b/proto/src/system_messages.proto
@@ -408,5 +408,9 @@
     // Notify the user about external display events related to screenshot.
     // Package: com.android.systemui
     NOTE_GLOBAL_SCREENSHOT_EXTERNAL_DISPLAY = 1008;
+
+    // Notify the user that accessibility floating menu is hidden.
+    // Package: com.android.systemui
+    NOTE_A11Y_FLOATING_MENU_HIDDEN = 1009;
   }
 }