Use GetConnectionCredentials method in gdbus helpers
Use GetConnectionCredentials method in gdbus helpers

This commit uses upstream GetConnectionCredentials method to obtain client
credentials instead of separate method for each credential type.
The benefit is twofold:
 - This method is included in upstream D-Bus
 - All credentials are obtained at once which might be helpful once credential
   caching is implemented

Change-Id: I6c378f7f87685d14e1d9924e724821a85bc3be88

Samsung/cynara,Samsung/cynara,Samsung/cynara