Make libhidl_gtest_helper vendor_available. am: d17de9f5a5 am: bad2fe7568 am: 7cebf0c8d4

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

Change-Id: Id39937e103b6cdab195578f2d22a4e9f7e1b1cd2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>