[Toybox] Android's SELinux commands are now all toybox!

[enh]

https://android-review.googlesource.com/#/c/145744/ (SHA
bbf0ba1ec7e2348d9f72265bac1f3faef867bffe) implements restorecon. this
completes the work of moving Android's SELinux command line tools over
to toybox.

rob will cherrypick the Android patches when he has time, but they're
available in AOSP in the meantime.

(he's not going to like this patch --- there's a #if. i couldn't think
of a better way to keep this building for the desktop too, which is
useful to me for testing purposes, even if no real desktop user is
going to want an Android-specific restorecon.)

 -e