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