Make copyright notices consistent

Use SPDX-License-Identifier copyright notices and apply consistently
to source throughout the repo. This covers all modules except
service. That one will be updated in a subsequent commit.

redis-dispatch/src/main/java/org/whispersystems/dispatch/DispatchManager.java

@@ -1,3 +1,7 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
 package org.whispersystems.dispatch;
 
 import org.slf4j.Logger;