Make libhidl_gtest_helper vendor_available. am: d17de9f5a5 am: 1501f57d6b

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

Change-Id: I04a25a656cbda3e39773b77cff204fb1d58c8830
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>