[Bug] TTS bugs

[qnga]

Describe the bug

• Going back from the end is completely broken (sentences are read "randomly"). ContentIterator and TtsPlayer probably don't conform to the same iterator specification.
• At least when reaching the end, Locations.after contains the last letter of the highlighted part.

How to reproduce?

1. Let the book finish.
2. Look at the utteranceWindow property for the location.
3. Call goToPreviousUtterance for the going back issue.

Readium version

develop

Android API version

30

Additional context

No response