[lib] Stop requesting device lists for users that don't have them

Summary:
issue: ENG-8842
When we launch CSAT, some users will not yet have device lists. We don't want to be requesting them every couple of seconds.

Test Plan:
Created a user registered with the keyserver but not with identity. Checked that their device list was being fetched multiple times, and none was returned by identity.
Checked that after this change the list was being requested only once.

Reviewers: tomek, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12793