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

(cherry picked from commit 438e0194)

Change-Id: I639f2142ad4ba049b990b13ccccd255be4b4f479