Made the bubble bar react to drag events from the Shell.

Added logic to show and hide the bubble bar drop target on Shell
drag-and-drop events.

Test: Manual. Drag a shortcut or an application icon from the taskbar to
the bubble bar drop zone. Observe that if the bubble bar is showing
bubbles, the background stroke color becomes non-transparent; if there
are no bubbles, the drop zone appears. Drag the app icon to the opposite
side and observe that the bubble bar location is animated or the drop
zone appears on the dragged side. Drop the application icon or shortcut
on any side and observe that a new bubble is added and immediately
expanded. Additionally, the bubble bar remains at the drop location.
expanded.
Bug: 388894910
Flag: com.android.wm.shell.enable_create_any_bubble

Change-Id: I581dddf2c76ac982a5c78bc32d94b79bf63fd2b9