[Passwords] Don't add POPUP_ITEM_ID_SEPARATOR on Android

Android doesn't have a notion of separators. (e.g. Keyboard accessory).
Separators are only relevant for Desktop.
This CL excludes the code that adds the separator on Android.

This code that adds the separated started to run on Android after
kAutofillVisualImprovementsForSuggestionUi has been enabled by default
in https://crrev.com/c/3870616

Change-Id: I53ea8eac50fcfc815e28e64a6765ba0ec2c3371f
Bug: 1247978
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3880322


Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Auto-Submit: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1044507}