fix: start tracking sessions at init for session replay

[justin-fiedler]

Summary

Sessions are not working as expected with the Session Replay functionality.

The main issue I am trying to solve is ensuring that amplitude.sessionId is set after isBuilt which was not always the case until now.

Additionally customers have asked for a way to listen to session changes without requiring session events. For this I updated the ObservePlugin to have onSessionIdChanged() callback that fires regardless of Session event tracking. Future versions of the Session Replay plugin can use this listener instead of listening for session events.

Since Session events can now be fired on instantiation e.g. val amp = Amplitude(...) I also added val amp = Amplitude(Configuration(plugins = listOf(..plugins that run on any events during init)). This can be used to attach params to session start and other potential DET events.

Full list of changes:

• No longer migrate session info (sessionId, lastEventTime) - since migration only happens on a new app install, this information can/should be reset.
• Move session logic into its own class
• Remove session logic from timeline
• Process session immediately in constructor, ensure amplitude.sessionId is ready before isBuilt is complete
• Add sessionId to state
• Add ObservePlugin.onSessionIdChanged callback
• support adding plugins to initial config, this is needed to add properties to session events created during initialization
• centralize time to use SystemTime for easier testing

Checklist

• [x] Does your PR title have the correct title format?
• Does your PR have a breaking change?: API - no, Functional - yes - session events now fire immediately rather than on the first event being send.

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[justin-fiedler]

I can see why sessionId is not guaranteed to be set after isBuilt as it's part of timeline. To fix this, instead of adding a session property in Amplitude, how about creating timeline in build()? So that we don't need to support adding plugins to initial config which is needed to add properties to session events created during initialization.

Thanks @Mercy811, I tried moving the session creation into the Timeline.start() but then I need to pass in all the dependencies from Amplitude (configuration, storage, store). I think this implementation is a little cleaner given that. wdyt?

[justin-fiedler]

Not sure why the one test is failing. It passes on my local. Debugging it now.

[justin-fiedler]

To not pass all the dependencies Amplitude (configuration, storage, store), how about accessing them via amplitude as core timeline already has amplitude as its class property.

Thanks @Mercy811 that made it a lot cleaner.