Add informational thread annotations to customer-facing methods

Based on the discussions @hokyungh and I had about [this issue](https://github.com/aws/amazon-chime-sdk-android/issues/260).
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](https://developer.android.com/reference/androidx/annotation/MainThread), [AnyThread](https://developer.android.com/reference/androidx/annotation/AnyThread) and [WorkerThread](https://developer.android.com/reference/kotlin/androidx/annotation/WorkerThread) accordingly.

Start by inventorying the SDK's customer-facing methods and reading their implementations to determine whether MainThread, AnyThread, or WorkerThread applies. The issue names no files or tests; done means the exposed methods carry appropriate AndroidX annotations and the project continues to build and pass its existing checks.

Written by the indexing model from the issue text.