Convert malloc_debug from Android.mk to Android.bp

Change-Id: I60ad376d222a92a29263c2804dcd1bd0d3c383d0
diff --git a/libc/Android.bp b/libc/Android.bp
index ba62592..7309bd8 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -2006,3 +2006,5 @@
 
     defaults: ["crt_defaults"],
 }
+
+subdirs = ["malloc_debug"]