Make libhidl_gtest_helper vendor_available. am: d17de9f5a5 am: bad2fe7568 am: c33821e9cb

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/libhidl/+/18453726

Change-Id: I43d7cdafa3b28d6ba84a1b3290bac7b6725785e5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>