Check conformity of VNDK version

Allow BOARD_VNDK_VERSION=<version> only if the prebuilt VNDK for
the <version> is provided.

If PRODUCT_EXTRA_VNDK_VERSIONS is set, all the versions in the list
will be checked as well.

Bug: 38304393
Bug: 65377115
Bug: 68123344
Test: Set BOARD_VNDK_VERSION := current and check build
Test: Set PRODUCT_EXTRA_VNDK_VERSIONS := 27 and check if it returns
      error when prebuilts/vndk/v27/Android.mk is not provided

Change-Id: I2e43d71f1ea3717ab8c45de84c926ecc43193307