Increase java heap size.

I got an Out of memory while compiling, Andreas Huber suggested that
increasing the heap size, which worked.

Change-Id: Id8293ef100ef814b0fe13aa6e1b891a36a2ee853
diff --git a/core/definitions.mk b/core/definitions.mk
index fa0705c..1b09d86 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1535,7 +1535,7 @@
 @echo "target Dex: $(PRIVATE_MODULE)"
 @mkdir -p $(dir $@)
 $(hide) $(DX) \
-    $(if $(findstring windows,$(HOST_OS)),,-JXms16M -JXmx1536M) \
+    $(if $(findstring windows,$(HOST_OS)),,-JXms16M -JXmx2048M) \
     --dex --output=$@ \
     $(incremental_dex) \
     $(if $(NO_OPTIMIZE_DX), \