Android: Settings background has a wrong background color in dark themes

Core::SettingsDialog puts the settings widget into a QScrollArea.

Commit faad83d5a37448b472af2efa76b7193c83f2c1f1 inserts another set of QScrollArea + QWidget under the whole form. This leads to two two of those and one is superfluous. When using a dark theme (e.g. flat-dark), this extra QScrollArea introduces wrong background color. Also, it complicates the already quite blown androidsettingswidget.ui.