Set default activity launchMode to singleInstance

This attriute is needed otherwise if the user puts the application in
background and uses the appication icon to bring it back in front,
andriod will create a new activity.

Task-number: QTBUG-37186
Change-Id: Icdc87239c1a07a3e555296692a4866eb6351348e
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>