Add the Product ID to target_files.zip

When using the A/B updater, include the product id string to
target_files.zip.
This is needed by the Omaha backend.

Bug: 24946811
Change-Id: I2ce5f6cffe440c29a1820348ee215c71cec07861
diff --git a/core/Makefile b/core/Makefile
index b1b3d13..0c9a825 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1607,6 +1607,7 @@
 	  echo "$${part}" >> $(zip_root)/META/ab_partitions.txt; \
 	done
 ifdef OSRELEASED_DIRECTORY
+	$(hide) $(ACP) $(TARGET_OUT_ETC)/$(OSRELEASED_DIRECTORY)/product_id $(zip_root)/META/product_id.txt
 	$(hide) $(ACP) $(TARGET_OUT_ETC)/$(OSRELEASED_DIRECTORY)/product_version $(zip_root)/META/product_version.txt
 endif
 endif