emoji_search: Fix duplicate emoji in multiple languages

Previously, emoji search results were naively concatenated together
based on the languages. This resulted in incorrect weighting orderings
and duplicate emoji.

Fix this by deduplicating seen emoji. As a result, the weightings
returned by emoji search may not necessarily be in sorted order, as a
low-weighting primary language emoji result will appear before a
high-weighting secondary language emoji result.

Fixed: b:359751669
Change-Id: I6a6a210070aa0be01ab45ea09b9e60866ff6b7b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5782685
Commit-Queue: Michael Cui <mlcui@google.com>
Reviewed-by: Jon Htin <jhtin@google.com>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1342113}