ohos: Handle text-to-speech states

This change implements handling for all relevant TTS states:
  * `Speaking` - set in `handleOnStart()`
  * `Ready` - set on speech completion or stop
  * `Error` - set on parsing errors or TTS errors

The `Paused` state is never set because the current OHOS JS TTS
API does not provide a `pause` function.

Task-number: QTBUG-147179
Pick-to: 6.12
Change-Id: Ida9ca060b522d484cff05296ce1ba60ce22e238c
Reviewed-by: Zbigniew Chyla <zbigniew.chyla@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>