Hide __system_property_add/update, deprecate find_nth

These functions are supposed to be used only by the
property service.

__system_property_find_nth is deprecated and no longer part
of NDK. Call to this function will result in abort for apps
targeting Android O.

Bug: http://b/34114501
Test: bionic-unit-tests --gtest_filter=prop*
Change-Id: I9846965bf248e2ddf45cd7b293618245bbd87145
diff --git a/libc/include/sys/system_properties.h b/libc/include/sys/system_properties.h
index d3e0d8d..faed9a0 100644
--- a/libc/include/sys/system_properties.h
+++ b/libc/include/sys/system_properties.h
@@ -81,7 +81,8 @@
 ** is inefficient and order of results may change from call
 ** to call.
 */ 
-const prop_info *__system_property_find_nth(unsigned n);
+const prop_info *__system_property_find_nth(unsigned n)
+  __REMOVED_IN(26);
 
 /* Pass a prop_info for each system property to the provided
 ** callback.  Use __system_property_read() to read the value