Parse multiple packages from Omaha response.

The multi-payload info are stored in OmahaResponse and InstallPlan, but
we still can only apply the first payload for now.

Bug: 36252799
Test: mma -j
Test: update_engine_unittests

Change-Id: I5ca63944ae9082670d0e67888409374f140d4245
(cherry picked from commit 2aba8a87d4fac245a2e2d238b3159f8eabce630f)
diff --git a/payload_consumer/download_action.h b/payload_consumer/download_action.h
index 285930a..0bd0d88 100644
--- a/payload_consumer/download_action.h
+++ b/payload_consumer/download_action.h
@@ -134,6 +134,9 @@
   // The InstallPlan passed in
   InstallPlan install_plan_;
 
+  // Pointer to the current payload in install_plan_.payloads.
+  InstallPlan::Payload* payload_{nullptr};
+
   // SystemState required pointers.
   PrefsInterface* prefs_;
   BootControlInterface* boot_control_;