Fix voltage warning announcements.
Fix voltage warning announcements.

AutoQuad/qgroundcontrol_aq,hejunbok/qgroundcontrol_aq,hejunbok/qgroundcontrol_aq,AutoQuad/qgroundcontrol_aq,hejunbok/qgroundcontrol_aq,AutoQuad/qgroundcontrol_aq,AutoQuad/qgroundcontrol_aq,hejunbok/qgroundcontrol_aq