[native] Show loading spinner during getOlmSessionInitializationData server call

Summary: This diff addresses ENG-4604.

Test Plan:

Note that there is a bit of a stutter when the first loadingSelector finishes, since the second one doesn't overlap with the first. I decided that this was okay for now... we'll end up needing to change things here pretty soon anyways, since these keys will get fetched from the identity service instead of the keyserver in the near future.

Reviewers: atul, marcin, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8762