Un-deprecated __system_property_find_nth.

Netflix was using this, and looking the header file, although
__system_property_find_nth has been available since the beginning of time,
__system_property_foreach only appeared in 16. So anyone who wants to run
on pre-JellyBean devices would want to use __system_property_find_nth.

It's pretty much a one-liner in terms of __system_property_foreach anyway,
so it doesn't cost us anything to keep it.

Also restore slightly better tests than we originally removed.

Bug: http://b/36566667
Test: ran tests
Change-Id: Id268c2c2e848da17bb0a5a5420af234d9dcb829a