GetAgreements: Include autoverify group in output

The auto verify group will be needed in the "new agreement" screen to
determine whether or not to enable the "submit" button.

Factor the initialization of the agreement info out to a utility class
so it can also be used by the "get server info" endpoint.

Change-Id: I8572e189b653a51310c630fcba20790bf70e5640

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/accounts/AgreementsIT.java

@@ -107,6 +107,8 @@ public class AgreementsIT extends AbstractDaemonTest {
     assertThat(info.name).isEqualTo(ca.getName());
     assertThat(info.description).isEqualTo(ca.getDescription());
     assertThat(info.url).isEqualTo(ca.getAgreementUrl());
+    assertThat(info.autoVerifyGroup.name)
+        .isEqualTo(ca.getAutoVerify().getName());
 
     // Signing the same agreement again has no effect
     gApi.accounts().self().signAgreement(ca.getName());