Check appservices for devices during a /user/devices query. (#15539)

MSC3984 proxies /keys/query requests to appservices, but servers will
can also requests devices / keys from the /user/devices endpoint.

The formats are close enough that we can "proxy" that /user/devices to
appservices (by calling /keys/query) and then change the format of the
returned data before returning it over federation.