Update GoogleSyncManagerUsernames to use the v201206 ManagedCustomerService logic

http://adwordsapi.blogspot.com/2010/10/discover-v201008-servicedaccountservice.html

• • "An important difference between AccountService.getClientAccounts (v13) and ServicedAccountService.get (v201109) is that getClientAccounts returns only the immediate child accounts, whereas ServicedAccountService.get() returns all of the accounts in the hierarchy."

Right now, googlesyncmanagerusernames app makes a separate API call for each MCC / Sub-MCC (aka manager), but each call gives the entire account hierarchy from that level as of v201109 upgrade. Previously only immediate children were returned, so children in a sub-mcc within the manager were not returned.. but now they are. However, the app expects only the immediate children to be returned, so the result returned from the API is shuffled through to only return the immediate children. The app still makes a separate call for each sub-mcc, but this is inefficient in v201109 because only one API call needs to be made since it now gets the entire account hierarchy. The logic in the app should be changed to reflect this.