Add variable PRODUCT_SHIPPING_API_LEVEL.

PRODUCT_SHIPPING_API_LEVEL declares which API level a device first
shipped with.

We use this variable to set a read-only property that can be used in
CTS/GTS tests.

Bug: 26381401
Change-Id: I9dfeaf423146c35f57e69ac26c27e8a9e205bc4f
diff --git a/core/product.mk b/core/product.mk
index 5b6a86c..94cc2e3 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -122,6 +122,7 @@
     PRODUCT_DEX_PREOPT_BOOT_FLAGS \
     PRODUCT_SYSTEM_BASE_FS_PATH \
     PRODUCT_VENDOR_BASE_FS_PATH \
+    PRODUCT_SHIPPING_API_LEVEL \