TTS Engine Extension: Emit UninstallLanguageRequest to extensions.

The approved API proposal:
https://docs.google.com/document/d/16C6iMB9lsnAZDKc6Ea2Ep-jZj23SOUgI5n62W-FI9gk/

Here is the relevant design doc: go/read-aloud-eng-design-phase3

Bug: 360395015
Change-Id: I44bba75c5633a8a3e5a2459278707c168ad3e530
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5954665


Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Eitan Goldberger <eitang@google.com>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378942}