[sync] Add new fields to ThemeSpecifics for theme attributes

This adds new fields in ThemeSpecifics for theme attributes which are
right now synced via preferences. If kMoveThemePrefsToSpecifics is
enabled, also reads/writes to the new fields.
Note: NtpCustomBackgroundDict will be added in a follow-up CL.

This also moves an existing optional field `autogenerated_theme` to a
new oneof for a more clear representation, which is safe as per
https://protobuf.dev/programming-guides/proto2/#updating.

Bug: 356148174
Change-Id: I7182ae769241347dd8ba3a999c3968ce9e551d9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5788405
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Ankush Singh <ankushkush@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1346789}