Make libhidl_gtest_helper vendor_available. am: d17de9f5a5 am: 0c4a397d7e

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

Change-Id: I5788d18336ff4cc628ca5329ef99bf622a20ccc2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>