Implementing FCM sendAll and sendMulticast APIs

[hiranya911]

Implementing the new APIs for sending messages in batches:

BatchResponse sendAll(List<Message>)
BatchResponse sendAll(List<Message>, boolean)
BatchResponse sendMulticast(MulticastMessage)
BatchResponse sendMulticast(MulticastMessage, boolean)

go/fcm-multicast for the full proposal.

Also implementing the following refactorings for clarity and modularity:

• Reducing the size of and responsibilities of FirebaseMessaging by introducing 2 new classes - FirebaseMessagingClient and InstanceIdClient.
• All the rpc logic is now implemented in the new classes, and FirebaseMessaging is just the public API for them.
• Moved some of the common API client logic to a new ApiClientUtils class, so we can share it across packages in the future.