Mark libmetricslogger vendor_available.

libmetricslogger is a library which belongs to vndk-cap. Mark it
vendor_available to enable vndk abi stability checks on it.

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

Test: mm -j64

Bug: 38244611

Change-Id: If4bb63d3769317bc7cfc0f2f326e8bdd7b565ebe