Add maximum timestamp to the payload.

Added a new field max_timestamp in the protobuf, from now on
update_engine will reject any payload without this field.
If the OS build timestamp is newer than the max_timestamp, the payload
will also be rejected to prevent downgrade.

Bug: 36232423
Test: update_engine_unittests

Merged-In: Ib20f5f35aaf41165013bada02bc8720917358237
Change-Id: Ib20f5f35aaf41165013bada02bc8720917358237
(cherry picked from commit 5011df68)
(cherry picked from commit ccb01b2a)
(cherry picked from commit 4c579f046e40306df1558b96308066f91c460787)
Exempt-From-Owner-Approval: OWNERS file outdated in nyc-*