TextArea placeholder text does not disappear on Android until word is committed

If correction suggestions (predicative text) enabled in keyboard settings, the text property not updated during typing, until the word is so called "committed". As placeholder visibility depends on text, both typed text and placeholder stays visible (see screenshot).

TextInput has no this problem, as it uses displayText to control visibility of the placeholder. Though it is too not updating text with typing, which may produce bad behavior in dialog.

Caused by QTBUG-49340.