FR: Add channel ID to AndroidConfig

[ambernardino]

Describe your environment

• Messaging/Notifications

Describe the problem

If you target Android 8.0 (API level 26) and post a notification without specifying a notification channel, the notification does not appear and the system logs an error.

If needed I can implement this feature.

[chemidy]

In the new HTTP V1 API android_channel_id is not present in the doc.

But in the legacy Server Protocol there was a android_channel_id.

Maybe an oversight ?

[hiranya911]

The FCM team is working on it.

[sampson-chen]

FCM team hopes to add this in the future for feature parity, but we do not have a timeline to share yet =)

[nadeemLGN]

Any updates on when this is going to be available on HTTP V1 API

[hiranya911]

This is now available in our REST APIs. Added to the SDK in #202