[REG 5.15.10 -> 5.15.11], PositionSource: last known position is never read

When an active PositionSource is created, it is not initialized with the last known position, because the if statement added in : https://codereview.qt-project.org/c/qt/qtpositioning/+/414974 can never be true. PositionSource only becomes active after setActive() has called start() through a QTimer. By then tryAttach() has already been run resulting in an invalid position on startup.

if (m_active) {
    const QGeoPositionInfo &lastKnown = m_positionSource->lastKnownPosition();
    if (lastKnown.isValid())
        setPosition(lastKnown);
}