[FSSDK-9918] add returnInMainThread flag for async init

[jaeopt]

Summary

This PR adds a flag (returnInMainThread) to async initialization for controlling to return on completion in main thread or background thread.

• by default, returnInMainThread = true for backward compatibility.
• set returnInMainThread = false to return in a background thread.

Test plan

• add new unit tests covering returnInMainThread on and off
• add new unit tests for backward compatibility (default returnInMainThread = true)

Issues

• FSSDK-9918