[keyserver] Use platform details in validateOutput

Summary:
Changes validateOutput to take platform details instead of a viewer. This allows us to use this function in places where we don't have the viewer, for example when validating the notifications.

Depends on D7786

Test Plan: yarn flow

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7805