Make libhidl_gtest_helper vendor_available. am: d17de9f5a5

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

Change-Id: I1f6a043b78c318368e8273f5cf4fce602641b7a0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>