Supporting Audio Overviews in ReadAloudController.

No functional change so long the feature flag is disabled.

Changes are:
1. Readability check includes AO readability info (as well as feature flag).
2. Set the requested playback mode correctly based on user preference, readability info, language.
3. Update mode selector enabled/disabled status based on whether both modes are supported.
4. Gate translation request for classic only.

Bug: 401256755
Change-Id: I85eb332b584011f37141e602b50ab66966c9e76a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6442699
Reviewed-by: Andrea Gomez <andreaxg@google.com>
Reviewed-by: Ian Wells <iwells@chromium.org>
Commit-Queue: Shlomi Ben Shimon <shlomib@google.com>
Cr-Commit-Position: refs/heads/main@{#1445664}