Fix a regression with availableSensorsChanged().
Fix a regression with availableSensorsChanged().

This signal is supposed to emitted once per (top-level) call
to QSensorManager::registerBackend(). It is also supposed to
be emitted only once after loading all plugins (assuming at
least one backend was registered).

Fixes: MOBILITY-2532

enthought/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility