ohos: Return the TTS proxy instead of a factory

tryMakeTextToSpeechProxyFactory() created one engine eagerly and
returned a std::function handing out that same shared_ptr, which read
as a factory while being a supplier of a fixed instance. Return the
proxy directly and promote setEngineEventsListener() to the
TextToSpeechProxy interface, so the engine attaches its listener after
construction.

Task-number: QTBUG-147179
Pick-to: 6.12
Change-Id: Ia907c1541c684f54b120885c861e4b427c2f5b44
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Dawid Śliwa <dawid.sliwa@qt.io>