[flutter_tts] Make speech speed proportional to speech rate

[swift-kim]

• Let setSpeechRate compute the speech speed (between min-max) based on the given normalized speech rate (0-1).
• Unsupport getSpeechRateValidRange which returns an invalid platform value ("android") on Tizen.
• Update flutter_tts to 3.5.0 and update the example app.
• Some code refactoring.
  • Remove the unused TTS error callback.
  • Let SetStateChanagedCallback return values of type TtsState.
  • Simplify comments.
  • Let HandleAwaitSpeakCompletion take an argument of type EncodableValue for consistency.
  • Inline OnStateChangedand OnUtteranceCompleted into relevant callbacks.

Previous discussion: #401 (comment)