Refactor the SDK to not assume that the seed is available without user interaction

[str4d]

Currently various public APIs of the SDK require passing the seed into them directly, making the assumption that the app can do so whenver that public API gets called (for example during Synchronizer initialization, we take the seed in order to perform any necessary wallet DB migrations). This won't be the case once we have hardware wallet support.

We need to ensure that:

• APIs the SDK expects the app to call automatically / whenever necessary do not require the seed.
• APIs that require the seed can be called either manually by the app, or have alternative pathways implemented and documented for use cases where the seed is not available on this device.