scanning: Add end margin inside settings dropdowns

Margin added to the end to match the margin in the start.

Before: http://screen/4rxikjreQEFFyzF
After: http://screen/8fdyGdR58egL2oY

Bug: 1059779
Change-Id: I01e9a6061e3d1d6efa1ccb43aa79e54e529840df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613784Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840823}

chromeos/components/scanning/resources/scanning_shared_css.html

@@ -20,6 +20,7 @@
       @apply --scanning-select-font;
       background-position-x: calc(100% - 12px);
       color: var(--scanning-select-text-color);
+      padding-inline-end: 32px;
       padding-inline-start: 16px;
     }