Part of ENG-10782.
Added a param to Inbound/Outbound keys RPC requests. The param specifies a white list of devices for which we want to retrieve keys.
On old clients built before this change, this field being missing is treated as an empty list. Treating empty lists as "unfiltered" will retain existing behavior and guarantee backwards-compatibility.
From proto docs about defaults:
The default value for repeated fields is empty (generally an empty list in the appropriate language).