[Read Aloud] Fix jumpiness on language menu.

This CL prevents re-rendering the language menu whenever
a toggle is pressed. This was causing the language menu to
"jump," causing a lot of visual noise and jank.

We no longer pass enabledLanguagesInPref into computeAvailableLanguages,
as this was forcing a re-render. When the menu is opened, we already
know which languages are enabled in preferences and don't need to
re-trigger a render each time a toggle changes.

Additionally, a "languageCode" property was added to the dropdown items,
so that all of the notification text and notification properties
can be updated directly, without triggering a re-render.

I verified that:
- Closing and reopening the language menu continues to show correct
  languages
- "Downloading voices" messages display correctly
- "No internet" install error notifications trigger correctly

Fixed: 337935335
Change-Id: I67b823c9e95913c194f6b3e324dc0297758ba415
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5499568
Reviewed-by: Kristi Saney <kristislee@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Cr-Commit-Position: refs/heads/main@{#1294106}