[Desktop][PwdCheck] Even icon paddings

With https://crrev.com/c/2260639, the paddings weren't distributed
evenly anymore (start/end padding) causing misalignments.
Additionally, the non-square icon dimensions caused rounding errors
increasing that effect.

This CL ensures consistent paddings for RTL and LTR mode. Screenshots
are in the linked bug.

Bug: 1144629
Change-Id: Ied4af63de6d94abb2b6510cfd061a970f8f98da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513600


Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823262}