Android: correctly update camera focus zones.

When setting a null custom focus point, we were correctly resetting
the focus zones but we were not emitting the focusZonesChanged signal,
causing the QML Camera to not update its focus zones.

Change-Id: I59940c4cd2979fa08dc26566dc0f6d19e503f64f
Reviewed-by: Christian Stromme <christian.stromme@digia.com>