why not using ther google account name? is easy to get and needs only a simple request on the manifest file. they will have purchased the license with gplay, so the g+ account name should be enough...
in the manifest:
<manifest ... >
<uses-permission android:name="android.permission.GET_ACCOUNTS" />
...
</manifest>
to retrieve the account name:
AccountManager am = AccountManager.get(this); // "this" references the current Context
Account[] accounts = am.getAccountsByType("com.google");
to retrieve the name:
accounts[0].name
i write a simple alert to make me sure i have found an account here the whole code:
Account[] accounts = am.getAccountsByType("com.google");
AlertDialog.Builder miaAlert = new AlertDialog.Builder(this);
miaAlert.setTitle("i found an account name!");
miaAlert.setMessage(accounts[0].name);
AlertDialog alert = miaAlert.create();
alert.show();