Server-side implementation of ENG-10782.
As mentioned in the Linear task, and D14758, treating empty list as unfiltered maintains previous behavior.

Depends on D14758

Used Kreya tool to call GetOutbound/InboundKeys RPCs. Confirmed that:

• Empty list returns keys for all user devices
• Specifying device IDs on the list makes RPCs return keys for only these devices. Verified in DDB that no OTKs are retrieved for filtered-out devices.