Make bionic_malloc.h a platform header.

Instead of having platform directories directly include the
private header, create a platform header directory and export it.

Bug: 130763340

Test: Builds.
Change-Id: Ie0f092b3fe077a3de8b90266c0b28bfbc20d0dfa
Merged-In: Ie0f092b3fe077a3de8b90266c0b28bfbc20d0dfa
(cherry picked from commit 8f582ef2f8a77d953d0e9f33387f592d20f852e2)
diff --git a/tests/malloc_test.cpp b/tests/malloc_test.cpp
index edcc179..989e2d0 100644
--- a/tests/malloc_test.cpp
+++ b/tests/malloc_test.cpp
@@ -33,8 +33,8 @@
 
 #include <android-base/file.h>
 
+#include "platform/bionic/malloc.h"
 #include "private/bionic_config.h"
-#include "private/bionic_malloc.h"
 #include "utils.h"
 
 #if defined(__BIONIC__)