Cross-platform: Player subscriber

[Rafael Campos]

Problem

The Player supports multi-flow. However, it can only pull new content, needing support for pushing content to it.

That's relevant for scenarios where the content needs to be pushed to the Player without relying on an action/transition (e.g. displaying streamed data).

Solution

Following the ManagedPlayer approach, implement a PlayerSubscriber wrapper that'll subscribe to events, swapping flow and content in reaction to those.