[Read Aloud] Rename variables from sentence to granularity.

Rename variables that should say "granularity" instead of "sentence."

Note that some things will still use "sentence" like GetNextSentence,
since that is the more accurate term in those contexts.

Bug:1474951

Change-Id: I80855b1e8026beac589d87b989d8d895c946557c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5181154
Reviewed-by: Eitan Goldberger <eitang@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Cr-Commit-Position: refs/heads/main@{#1244772}