Add vendor_available to libz

This, when used with BOARD_VNDK_VERSION flag, enables the
module to be built and installed twice, one for /system,
other for /vendor

Details: https://android-review.googlesource.com/368372

Bug: 33241851
Test: Set BOARD_VNDK_VERSION:=current in BoardConfig.mk
Change-Id: Ibf4dcc34ba73834c5d9a19412be90724f94e0a6d