JSON support in data node while sending notification for Android through Firebase Admin SDK

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### [REQUIRED] Step 2: Describe your environment

* Operating System version: Windows 10 Enterprise, 1803
* Firebase SDK version: 6.8.0
* Library version: _____
* Firebase Product: Cloud Messaging (auth, database, storage, etc)

### [REQUIRED] Step 3: Describe the problem
The Firebase message object takes Map type for the data node. There is no support to send Map in the data node. With the legacy GCM protocol, it was possible to send multilevel map/JSON in the data node. Having this behaviour changed, would break the clients. If we convert the Object to String, this would break the clients.