vector: Apply coding conventions to screen.less and minor cleanup

vector: Apply coding conventions to screen.less and minor cleanup

Follows-up 08cf0b1, 7e953bbe.

* Removed obsolete comment about @embed, since this file no longer
  uses that.

* Removed ".less" per LESS convention.

* Made whitespace consistent with coding conventions.
  No spaces inside .transition(), just like there are no spaces
  in .background-image() or url().

* Line breaks between different rules.

Change-Id: I5956d179eb46f6ee4e0c313eafe008c434e61e31