[Read Aloud] Fix toast contrast

Show the language downloaded toast in front of language menu dialog
when it's open so it does not get partially blocked
by the dialog scrim.

This is done by adding a cr-toast element inside the dialog and show that one when the dialog is open.

https://screenshot.googleplex.com/6pxGi6Uv8ymYkjq

Bug: 343966559
Change-Id: I9dec7bc2a23553c28bce72ec14104c67cd9dcd99
Fixed: 343966559
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5607344


Reviewed-by: Lauren Winston <lwinston@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Reviewed-by: Eitan Goldberger <eitang@google.com>
Cr-Commit-Position: refs/heads/main@{#1312820}