Revert "Use colorPrimary at 25% opacity for Read Aloud highlight"

This reverts commit 18bd16713e9b2691969001cc7819a093f721f792.

Reason for revert: Causing test failures on https://ci.chromium.org/ui/p/chrome/builders/ci/chromium-internal-arm-builder

Original change's description:
> Use colorPrimary at 25% opacity for Read Aloud highlight
>
> Screenshot: https://screenshot.googleplex.com/BnxRSh84MbtPWMF.png
>
> Bug: 421249056
> Change-Id: Id7fc089937769d357d00ca1f472888e63a4dd021
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7006873
> Commit-Queue: Andrea Gomez <andreaxg@google.com>
> Auto-Submit: Ian Wells <iwells@chromium.org>
> Reviewed-by: Andrea Gomez <andreaxg@google.com>
> Cr-Commit-Position: refs/heads/main@{#1524355}

Bug: 421249056
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ibec5995dd563bae641490ace1edbba1788089c49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7006330
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524408}