[Read aloud] Clear initial sentence highlight after word boundary

When first playing with word highlighting on a page, or when quickly
clicking next, we don't know for sure if word boundaries are supported,
so sentence highlight is used. Once we receive the word boundary, we
then start word highlighting, but the sentence is now faded out as
'previous' which is inaccurate. This CL ensures that sentence highlight
is cleared instead.

Bug: 438495945
Change-Id: Id71bff34a14dffd83d3c449852385a52b500c3fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6907514
Commit-Queue: Kristi Saney <kristislee@google.com>
Reviewed-by: Lauren Winston <lwinston@google.com>
Cr-Commit-Position: refs/heads/main@{#1509929}