[native] Update KeyserverSelection to take URL instead of username as input
Summary:
Minor changes to indicate to the user that we are expecting them to input a URL.
Depends on D8193
Test Plan:
- Confirm placeholder appears correctly
- Confirm on-screen keyboard includes ".com" key on iOS simulator
Reviewers: atul, ginsu
Reviewed By: ginsu
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8194