[directorytree] Add files-ng 20px SVG volume icons

 - renamed the input files to match the names of the existing volume
   PNG image icon files used in Files app
 - tested that these icon could be used as CSS -webkit-mask, or else
   filled with a CSS specified (fill/background) color
 - use <svg width="20" height="20" ... > everywhere
 - viewBox is 0 0 20 20 as a rule: however, two icons have 0 0 24 24
   viewBox to be updated later by UX (computer.svg, shared.svg)
 - optimized using the SVGO in chrome/src with --precision 4

Bug: 992819, 1002393
Change-Id: Ibeb42bbcb490c4263d8944f1b0339b5cbeb986c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911346


Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714431}