Set BOARD_VNDK_VERSION=current as a default value

For Android P or later launching devices, set BOARD_VNDK_VERSION to
"current" as a default value if the value is not defined yet.
"PRODUCT_USE_VNDK_OVERRIDE=false" can disable this feature.

Bug: 70480159
Bug: 62019611
Test: Unset BOARD_VNDK_VERSION
      Set PRODUCT_SHIPPING_API_LEVEL "28"
      - Check if BOARD_VNDK_VERSION is set to "current"
      Set PRODUCT_USE_VNDK_OVERRIDE to "false"
      - Check if BOARD_VNDK_VERSION is not defined

Change-Id: I8d37579e8c4ca1788705cbc8a42e8812e2b93a22