Refresh PEP on session establish

We now track preKeys used to establish incoming sessions with us. On
each new established session, we remove the used prekey from PEP. We
have to do this because libaxolotl-java internally clears the used
preKey from its storage, so we will not be able to establish any future
sessions using that key.

src/main/java/eu/siacs/conversations/crypto/axolotl/AxolotlService.java

@@ -444,6 +444,7 @@ public class AxolotlService {
 	public static class XmppAxolotlSession {
 		private SessionCipher cipher;
 		private boolean isTrusted = false;
+		private Integer preKeyId = null;
 		private SQLiteAxolotlStore sqLiteAxolotlStore;
 		private AxolotlAddress remoteAddress;
 
@@ -463,6 +464,14 @@ public class AxolotlService {
 			return this.isTrusted;
 		}
 
+		public Integer getPreKeyId() {
+			return preKeyId;
+		}
+
+		public void resetPreKeyId() {
+			preKeyId = null;
+		}
+
 		public byte[] processReceiving(XmppAxolotlMessage.XmppAxolotlMessageHeader incomingHeader) {
 			byte[] plaintext = null;
 			try {
@@ -470,6 +479,9 @@ public class AxolotlService {
 					PreKeyWhisperMessage message = new PreKeyWhisperMessage(incomingHeader.getContents());
 					Log.d(Config.LOGTAG, "PreKeyWhisperMessage ID:" + message.getSignedPreKeyId() + "/" + message.getPreKeyId());
 					plaintext = cipher.decrypt(message);
+					if (message.getPreKeyId().isPresent()) {
+						preKeyId = message.getPreKeyId().get();
+					}
 				} catch (InvalidMessageException | InvalidVersionException e) {
 					WhisperMessage message = new WhisperMessage(incomingHeader.getContents());
 					plaintext = cipher.decrypt(message);
@@ -946,6 +958,12 @@ public class AxolotlService {
 					Log.d(Config.LOGTAG, "Got payload key from axolotl header. Decrypting message...");
 					plaintextMessage = message.decrypt(session, payloadKey);
 				}
+				Integer preKeyId = session.getPreKeyId();
+				if (preKeyId != null) {
+					publishBundlesIfNeeded();
+					session.resetPreKeyId();
+				}
+				break;
 			}
 		}