Make jemalloc the default choice.

Change-Id: Iadf29d28758bc17904098b4eeb9bc14a0a51299e
diff --git a/runtime/Android.mk b/runtime/Android.mk
index 7f5cf0c..b0c1a9c 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -309,10 +309,10 @@
   LIBART_CFLAGS += -DART_USE_PORTABLE_COMPILER=1
 endif
 
-ifeq ($(MALLOC_IMPL),jemalloc)
-  LIBART_CFLAGS += -DUSE_JEMALLOC
-else
+ifeq ($(MALLOC_IMPL),dlmalloc)
   LIBART_CFLAGS += -DUSE_DLMALLOC
+else
+  LIBART_CFLAGS += -DUSE_JEMALLOC
 endif
 
 # $(1): target or host