Added a toolbar in glanceable hub for editing

- Added a toolbar above the grid with three buttons for add/remove/done.
- Drag and drop a widget into the boundaries of the remove button
  will remove the item.
- TODO: clicking on Done button should go back to glanceable hub.

Bug: b/315154364
Test: manually
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Change-Id: Ib92026e9f43e0da36a50099bcb41f73c4cae7de9