emoji-picker: generalise emoji history management

To allow future refactor of the emoji history and unifying the data
presentation of different categories, part of refactoring emoji history
is conducted:

* History codes are generalised to allow addition of any new categories
  without requiring much code change.
* Source of history data for both UI and internal use is unified.
* HTML element names are standardised for different categories.
* History related operations are encapsulated through a sets of
  functions. Outside these functions, no parts require knowing
  implementation details of history management or their data structure
  making future refactors easier.
* Incognito state is handled more accurately and it relevant logics are
  optimized.
* Storage of preference is improved.


Fixed: b:231500029
Change-Id: Iec61ef9c53e34160a261918952a94a5a1763d27f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3695729
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: John Palmer <jopalmer@chromium.org>
Commit-Queue: Mehrab N <mehrab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1019368}