Polish notification dismiss button

This CL polishes the notification dismiss button that shows behind
the flag. In detail:

1. The notification dismiss button is located on the top-end corner.
2. The button shows when the host notification is under hover;
  otherwise, the button is gone.
3. When the notification dismiss button shows, the notification
  background shows a cutout.

Bug:b/365585705
Flag:com.android.systemui.notification_add_x_on_hover_to_dismiss
Change-Id: I82e96c7e12d07929242e0c0033e8562cb58fbfd9