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.

gcm-sender-async/src/test/java/org/whispersystems/gcm/server/MessageTest.java

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

gcm-sender-async/src/test/java/org/whispersystems/gcm/server/SenderTest.java

@@ -1,3 +1,7 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
 package org.whispersystems.gcm.server;
 
 import com.fasterxml.jackson.annotation.JsonAutoDetect;

gcm-sender-async/src/test/java/org/whispersystems/gcm/server/SimultaneousSenderTest.java

@@ -1,3 +1,7 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
 package org.whispersystems.gcm.server;
 
 import com.fasterxml.jackson.annotation.JsonAutoDetect;

gcm-sender-async/src/test/java/org/whispersystems/gcm/server/util/FixtureHelpers.java

@@ -1,3 +1,7 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
 package org.whispersystems.gcm.server.util;
 
 import com.google.common.base.Charsets;
@@ -40,4 +44,4 @@ public class FixtureHelpers {
       throw new IllegalArgumentException(e);
     }
   }
-}
+}

gcm-sender-async/src/test/java/org/whispersystems/gcm/server/util/JsonHelpers.java

@@ -1,3 +1,7 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
 package org.whispersystems.gcm.server.util;
 
 import com.fasterxml.jackson.core.JsonProcessingException;