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 5011df680621eb477cad8b34f03fba5b542cc2f9)
diff --git a/hardware_android.h b/hardware_android.h
index 78af871..80fd9df 100644
--- a/hardware_android.h
+++ b/hardware_android.h
@@ -47,6 +47,7 @@
   bool CancelPowerwash() override;
   bool GetNonVolatileDirectory(base::FilePath* path) const override;
   bool GetPowerwashSafeDirectory(base::FilePath* path) const override;
+  int64_t GetBuildTimestamp() const override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HardwareAndroid);