sc (low prio): schedule background job to refresh prekeys if needed every 2 days
context
- as of !578 (merged) , signalc checks to refresh prekeys every time it receives a new prekey bundle
- in addition to this, signal-android also runs a background job to check to see whether refreshing is necessary every 3 days. not entirely clear why they do this, but presumably has something to do with creating a regular heartbeat to notice whether signed prekeys have been rotated as per #487