libbinder: RPC test detect vsock support

Two reasons:
- b/186785704 (devices w/o vsock_loopback kernel module shouldn't have
  this requirement in VTS)
- b/185269356 (testing infra on host can't test vsock_loopback, but
  if we locally enable vsock_loopback, we want it to run)

Bug: --^
Test: \
  sudo modprobe vsock_loopback
  ..../binderRpcTest # works w/ vsock
  sudo modprobe -r vsock_loopback
  ..../binderRpcTest # only tests other socket types

Change-Id: Ieaeee990843aafdf6baedadd2f26177d53feeb96