Merge <sys/system_properties.h> and <sys/_system_properties.h>.

You could argue that some of this stuff (such as the constants used for communication between init and libc that _aren't_ passed on to callers) should be moved to a new <platform/system_properties.h> instead, but in the sense of "exposed to the NDK" rather than "useful to NDK users", all of this is already technically public API.

We can worry about how to make the platform-only implementation details less visible in documentation if/when we ever actually have the bionic docs generated and published on the web...

Change-Id: I1a6456cf7d301753ec8f541a2046538cd59ed58a
diff --git a/tests/system_properties_test.cpp b/tests/system_properties_test.cpp
index f11f509..5b5e009 100644
--- a/tests/system_properties_test.cpp
+++ b/tests/system_properties_test.cpp
@@ -33,10 +33,9 @@
 
 #if defined(__BIONIC__)
 
-#define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_
 #include <stdlib.h>
-#include <sys/_system_properties.h>
 #include <sys/mount.h>
+#include <sys/system_properties.h>
 
 #include <system_properties/system_properties.h>