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: If269fe79618ce2c0237d09db78d80f06bbdc0e01
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>