Update SystemUpdatePolicy javadoc.

This is a pure javadoc update to reflect the fact that security updates
are no longer affected by the postponement policy. The CL also includes
rewording of some existing javadoc to clarify the expiration of local
policies.

Test: none
Bug: 33101785
Change-Id: If8c9947a41b4bac9c143613a940a62fee54879fc