add amCustodian method which determines whether you are a custodian for a given contact

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/SocialBackupManager.java

@@ -1,5 +1,6 @@
 package org.briarproject.briar.api.socialbackup;
 
+import org.briarproject.bramble.api.contact.Contact;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
@@ -52,4 +53,6 @@ public interface SocialBackupManager extends
 	@Override
 	Collection<ConversationMessageHeader> getMessageHeaders(
 			Transaction txn, ContactId contactId) throws DbException;
+
+	boolean amCustodian(Transaction txn, ContactId contactId);
 }

briar-core/src/main/java/org/briarproject/briar/socialbackup/SocialBackupManagerImpl.java

@@ -230,6 +230,17 @@ class SocialBackupManagerImpl extends ConversationClientImpl
 		return false;
 	}
 
+	public boolean amCustodian(Transaction txn, ContactId contactId) {
+		try {
+			GroupId groupId = getContactGroup(db.getContact(txn, contactId)).getId();
+			return findMessage(txn, groupId, SHARD, false) != null;
+		} catch (DbException e) {
+			return false;
+		} catch (FormatException e) {
+			return false;
+		}
+	}
+
 	@Nullable
 	@Override
 	public BackupMetadata getBackupMetadata(Transaction txn)

briar-core/src/test/java/org/briarproject/briar/socialbackup/SocialBackupIntegrationTest.java

@@ -122,6 +122,9 @@ public class SocialBackupIntegrationTest
 			ShardMessageHeader s = (ShardMessageHeader) h;
 			assertFalse(s.isLocal());
 		}
+        db1.transaction(false, txn -> {
+        	assertTrue(socialBackupManager1.amCustodian(txn, contactId0From1));
+        });
 
 		// the shard message from 0 should have arrived at 2
 		Collection<ConversationMessageHeader> messages0At2 =