Make the tappable area of the reference drawer close button bigger

Alpha and Beta only. According to the new design specs we want at least
44x44px tappable area. Icons are implemented in a way that they have
the left and right margins of 1em, but not the top and bottom margins.
Adding top and bottom paddings should make the close icon a square.

Bug: T92700
Change-Id: Iccb6b67f483e66ec687695280e7779334b722e54