Make libhidl_gtest_helper vendor_available. am: d17de9f5a5

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

Change-Id: I7085a37ddb58c42c285061a53e21c0501cd9cb62
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>