[Read Aloud] Play / stop speech when the play button is toggled.

This is not a perfect implementation and is just intended to get some
basic functionality in place in order to unblock more work.

I added TODOs in as many places as possible where I know things
definitely need to change.

Bug: 1474951
Change-Id: I2b910a41eaa35f2c083568c1266ab4831835c26a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4808491
Commit-Queue: Lauren Winston <lwinston@google.com>
Reviewed-by: Kristi Saney <kristislee@google.com>
Cr-Commit-Position: refs/heads/main@{#1187536}