Files app: Reposition indicator on save/open dialog

Changes the padding on the select file type menu drop down to allow for
16px either side of the text. Allows 20px for the drop down indicator
itself. Positions the drop down indicator in the center of its 20px
allocated padding.

Note: original paddings matched the native select control placement. The
new text padding follows the Chrome OS text button guidance.

Bug: 1110886
Tests: Manually tested, in LTR and RTL modes.
Change-Id: Ieb8bb71f2057cae83c1826985d84be671719e793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3140869


Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918492}