ohos: Query the available voices without an engine

Core Speech Kit exposes listing the voices both as a method of a created
engine and as a module function. Use the module function, so the voices
can be queried before an engine exists.

This is a preparatory step: the timbre can only be chosen in
createEngine(), so selecting a voice will mean creating the engine for
that voice, which requires knowing the voices first.

Task-number: QTBUG-149561
Pick-to: 6.12
Change-Id: I5f9b49bfc46f12a72458377f3b16d13e0d2fe068
Reviewed-by: Dawid Śliwa <dawid.sliwa@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Zbigniew Chyla <zbigniew.chyla@qt.io>