Add Cut/Copy/Delete operation to currently focused item.

Also removed ability to focus on items in the directory list while
there's an on-going selection to avoid having a weird state of both
focus and selection. Shift extension still works, just that once user
puts focus on RootsFragment, they won't be able to focus back into
DirectoryFragment.

Bug: 32141890
Change-Id: I14c06f8616b63d04f0b5f489032f7006f91416a8