tls-psk: return identity hint as client identity

this is necessary for openssl, which expects "Client_identity" as
default client identity hint

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/network/KeyTransferInteractor.java

@@ -413,6 +413,16 @@ public class KeyTransferInteractor {
             this.presharedKey = presharedKey;
         }
 
+        @Override
+        public String chooseClientKeyIdentity(String identityHint, Socket socket) {
+            return identityHint;
+        }
+
+        @Override
+        public String chooseClientKeyIdentity(String identityHint, SSLEngine engine) {
+            return identityHint;
+        }
+
         @Override
         public SecretKey getKey(String identityHint, String identity, Socket socket) {
             return new SecretKeySpec(presharedKey, "AES");