Verify identity keys on outgoing messages.

If PreKeyEntity identity key doesn't match local DB, fail
outgoing message and queue "incoming" identity key update
message for manual user approval.

src/org/thoughtcrime/securesms/sms/IncomingIdentityUpdateMessage.java

@@ -0,0 +1,18 @@
+package org.thoughtcrime.securesms.sms;
+
+public class IncomingIdentityUpdateMessage extends IncomingKeyExchangeMessage {
+
+  public IncomingIdentityUpdateMessage(IncomingTextMessage base, String newBody) {
+    super(base, newBody);
+  }
+
+  @Override
+  public IncomingIdentityUpdateMessage withMessageBody(String messageBody) {
+    return new IncomingIdentityUpdateMessage(this, messageBody);
+  }
+
+  @Override
+  public boolean isIdentityUpdate() {
+    return true;
+  }
+}