Add system/${LIB}/vndk to default namespace for vendor.

With BOARD_VNDK_VERSION=current, vndk libs will be installed in
system/${LIB}/vndk. To make them available for vendor, it must be
added to default namespace.

Bug: 63866913
Test: build and boot with BOARD_VNDK_VERSION=current
Change-Id: I9e467a6125fc89513754b56a2420975559144f98