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
diff --git a/libc/include/sys/system_properties.h b/libc/include/sys/system_properties.h
index b55566e..c0bd445 100644
--- a/libc/include/sys/system_properties.h
+++ b/libc/include/sys/system_properties.h
@@ -91,11 +91,11 @@
 /* Deprecated. In Android O and above, there's no limit on property name length. */
 #define PROP_NAME_MAX   32
 /* Deprecated. Use __system_property_read_callback instead. */
-int __system_property_read(const prop_info *pi, char *name, char *value);
+int __system_property_read(const prop_info* pi, char* name, char* value);
 /* Deprecated. Use __system_property_read_callback instead. */
-int __system_property_get(const char *name, char *value);
-/* Deprecated. Use __system_property_foreach instead. Aborts in Android O and above. */
-const prop_info *__system_property_find_nth(unsigned n) __REMOVED_IN(26);
+int __system_property_get(const char* name, char* value);
+/* Deprecated. Use __system_property_foreach instead. */
+const prop_info* __system_property_find_nth(unsigned n);
 
 __END_DECLS