[PROPOSAL] Refactor getters to functions

[Ascenio]

There are these two cases in AgoraClient that getters are used to get async information.

https://github.com/AgoraIO-Community/Flutter-UIKit/blob/4ab388978f322caf99234420f5e8f510ebcd0a34/lib/src/agora_client.dart#L15-L30

This is often thought as not good practice because it tricks the client into thinking that it's a cached or O(1) operation. This is not recommended by Flutter guideline.

So maybe it should be changed to a function to make its computation more explicit.

[tadaspetra]

Changes merged in #16