Make libhidl_gtest_helper vendor_available. am: d17de9f5a5 am: bad2fe7568

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

Change-Id: Ifc4702ccf0ea93714969e49506d2e5c707bd9d12
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>