Taskbar drag starts internal pre-drag before system drag

- TaskbarDragController now extends DragController.
- Currently there is no pre-drag condition, so we immediately get onDragStart(), which starts the system global drag (which cancels the original internal drag).
- Make the original view invisible during the drag and drop operation, across both internal and system drag events.
- No longer handle onDragEvent() in TaskbarView, as TaskbarDragController handles all of it now.

Test: Drag and drop from taskbar still works (bonus: starts from the correct registration point that you touched down on). Locally added a PreDragCondition and verified a seamless handoff to system drag and drop when the pre drag end condition was met.
Bug: 182981908
Change-Id: I6bf48141a5eedfc6db6f461258e880ef8146e733