Sane ownership/permissions for directories in /data/nativetest{,64}.

This makes native test directories 750 root:shell. This matches the
value for files within those directories, and results in a more usable
access paradigm when (say) unprivileged test tools need to discover
these test binaries.

Bug: 25668833
Change-Id: I9bd2081b2c211b4383b5873238aaf64597756714