[native] Use loading button variant in ConnectEthereum during username search
Noticed I forgot to do this while working on D7925. See videos in Test Plan to see the impact.
Depends on D7926
To make the loading state clear, I added an artificial 2s delay to the username search keyserver endpoint.
Reviewers: atul, ginsu
Reviewed By: atul
Differential Revision: https://phab.comm.dev/D7927