feat: Associate replays with errors and traces on Android

[jamescrosswell]

Part of #2136

Associate Errors and Traces with the active Session Replay (if one exists) on Android.

Protocol

Although there is a new Replay context, I found that just setting the replay_id on that context (e.g. for an event) did not result in the event showing in the errors pane for session replays.

What was recommended, and work better, is to store the replay_id on the DSC (which will accompany all events) and let Relay unpack it from there. The DSC specification mentions replay_id as an optional property on the dynamic sampling context.

Getting the Replay ID

There are three ways the DSC can be created:

1. From the currently active transaction
2. From the propagation context
3. From a BaggageHeader (in distributed tracing scenarios)

In all 3 cases, we check to see if there's a Session Replay actively being recorded. If there is, we either set (or overwrite) the replay_id that int he DSC. If there's not, we just propagate whatever is in the header.

This is a bit different to what we normally do with the baggage headers, so there are quite a few new scenarios to unit test.

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