[Messaging] Make BigPictureStyle optional behaviour

## What feature would you like to see?

Currently when setting the `image` property on the [Notification object](https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages#Notification), the image is [shown unconditionally](https://github.com/firebase/firebase-android-sdk/blob/master/firebase-messaging/src/main/java/com/google/firebase/messaging/DisplayNotification.java#L147) using the `BigPictureStyle` notification style.

Having some kind of API to either disable or change this behaviour is a requirement for us.

Currently we have to resort to [patching](https://github.com/getdelta/firebase-android-sdk/commit/0e452d9a3814e0fc75c9323f4866cd43c8bae52f) the Firebase library at buildtime, which is not an ideal nor maintainable situation.

## How would you use it?

We would use this feature by disabling the `BigPictureStyle` and showing the image only as a smaller icon, not a full picture.

## Notes

On iOS we have to use a similar hack, but this is more maintainable as iOS provides a nicer way to hook into manipulating the notification before it is shown. So if this would get implemented on Android, having something similar on iOS would be nice as well.