make libasync_safe vendor_available:true

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

libasync_safe is used by libbacktrace which is in turn used by libs in
/vendor and thus marked as vendor_available. Therefore, libasync_safe
is also marked as vendor_available.

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

Bug: 33241851
Test: BOARD_VNDK_VERSION=current m libasync_safe.vendor successful

Merged-In: I4b4de3b99a024d52612109cee3d66b4e5fc12dec
Change-Id: I4b4de3b99a024d52612109cee3d66b4e5fc12dec
(cherry picked from commit ec0d6b41)