[ChromeVox] Move AbstractTts to background/

Clarify that all TTS state lives in the background context by moving
AbstractTts into the background/ directory.

This change is a pure refactor.

AX-Relnotes: n/a.
Bug: b:260599332
Test: cq
Change-Id: Ide563a113e1cebb343ee2abed24bb2954cc8434b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4053884


Reviewed-by: Josiah Krutz <josiahk@google.com>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/main@{#1076561}