Document how to select a specific Android theme

Change-Id: If6d72a65b01453e2f2ea76026ec158c98e80421e
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>