Update the Syncrhonizer API to use StateFlow liberally

[gmale]

As a wallet developer, I'd like to use StateFlows as much as possible because they are a better replacement for livedata (and similar workflows) than Flows.

Criteria:

• pretty much every Flow in the Synchronizer interface should be replaced with a StateFlow as was recently done with balances

Background:

• Originally most things in the Synchronizer API were channels
• When flows came out we switched over because they are an improvement over channels, which should be considered as communication primitives (i.e. not something to expose everywhere) per the various writings of Roman Elizarov the Kotlin/Coroutine Lead.
• StateFlows now exist and are a significant improvement and simplification. They can be considered a direct replacement for LiveData (without the lifecycle awareness, but that can be gained by structured concurrency)

[ccjernigan]

This migration was done already.