Enable vndk_package if BOARD_VNDK_VERSION is set

This is a first attempt to enable build system support to include
vndk_package automatically (if BOARD_VNDK_VERSION is set), so devices
don't need to add it via PRODUCT_PACKAGE manually.

Bug: b/67002788
Test: Booted on the x86 emulator and a physical device and checked
      their /system/lib*/vndk-* directories for vndk_package libraries.
    # The commands below are for the x86 emulator:
    $ lunch aosp_x86_64-userdebug; m -j
Change-Id: Iea927b9535c7ee1b64ca130a4ac54f0fd8b3f758
diff --git a/core/main.mk b/core/main.mk
index 5675f90..86b8a8f 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -891,6 +891,9 @@
   # by the appropriate product definition file, which was included
   # by product_config.mk.
   product_MODULES := $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_PACKAGES)
+ifdef BOARD_VNDK_VERSION
+  product_MODULES += vndk_package
+endif
   # Filter out the overridden packages before doing expansion
   product_MODULES := $(filter-out $(foreach p, $(product_MODULES), \
       $(PACKAGES.$(p).OVERRIDES)), $(product_MODULES))