[Read Aloud] Disable play / pause button when content is loading.

Based on UX guidance.

Bug: 325102951
Fixed: 325102951
Change-Id: I1aa4e2ac2a86815a8e3d0ebcf81f3baacf15bfa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5357159


Reviewed-by: Kristi Saney <kristislee@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Cr-Commit-Position: refs/heads/main@{#1270526}