[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