CSRF tokens should only be required (and only be allowed) if
the current session isn't already inherently safe against
CSRF due to the way the authentication mechanism works.
This allows (and requires) tokens to be omitted for requests
that use an OAuth Authorization header.
Bug: T230843
Bug: T230842
Bug: T237852
Change-Id: Ib2922d556ff2470d4bf8c386c18986ca9f37d1b5