Add informational thread annotations to customer-facing methods

[tmilovan123]

Based on the discussions @hokyungh and I had about this issue.
Sometimes some of the exposed methods might do a lot of work on the UI thread, and moving them off UI thread can resolve certain issues.
However, as a consumer of the SDK, we cannot be sure a certain method is safe to be called off UI thread or not. It can be determined by reading the code sometimes, but this approach requires some time and effort.
Suggestion: annotate publicly facing methods with MainThread, AnyThread and WorkerThread accordingly.