Mark libion vendor_available.

libion 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

Merged-In: I195145af90433c29ce9e408197e8d7183cb960bc
Change-Id: I8ac290d435b7c4dc5e669bfccb4562495ce92516
(cherry picked from commit 2e31a8c4)