Make libhidl_gtest_helper vendor_available. am: d17de9f5a5 am: 0c4a397d7e am: 2a0305dbee

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

Change-Id: I4fdec5c5e5e96d8439c314e332891c453b378a93
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>