emoji-picker: unifying category data presentation

Continuing some recent refactors, category data presentation is unified and the following improvements are applied:

* The separate data sources for different categories are removed and replaced by one single source of data where category is only an attribute of each record.
* Logics for rendering of different categories are centralised and unified.
* Rendering of history groups are now handled the same way as any other emoji groups rather than a special case.
* EmojiGroupElements are created as a transformation of emoji group data helping to extend UI features of emoji groups easier and independent from other features such as search.
* Minor changes to metadata is applied and some unused codes are removed.

Fixed: b:235418846,b:233270330
Change-Id: I81cc05dd535b3e98f3f07225fc62045e3432459a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3695832
Reviewed-by: Keith Lee <keithlee@chromium.org>
Commit-Queue: Mehrab N <mehrab@chromium.org>
Reviewed-by: John Palmer <jopalmer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1019372}