[Read aloud] Fix highlighting when switching locales while reading

When the voice changes while reading, we stop speaking and then we send
the tts engine the rest of the sentence starting from the beginning of
the word that was last being read. The engine then sends us word
boundaries starting from the beginning of the text (i.e. index 0). This
is fine when we on the read aloud side are keeping track of where we are
in the original sentence. However, we were resetting all that
information when switching voices across locales because the new voice
might not support word boundaries.

Instead of resetting the whole state, this CL resets only the mode so
that if a boundary is detected with the new voice, we can keep our
place in the overall sentence. If the new voice does not support word
boundaries, the word boundary indices are irrelevant and will be reset
on the next sentence.

Fixed: 406809469
Change-Id: I7d938643a07c392b33e22e9ecd21676650efdc4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6825387
Reviewed-by: Lauren Winston <lwinston@google.com>
Auto-Submit: Kristi Saney <kristislee@google.com>
Commit-Queue: Kristi Saney <kristislee@google.com>
Cr-Commit-Position: refs/heads/main@{#1497880}