art: fix host dex2oat runtime args

The runtime arguments need to be prefixed with -Xmx or -Xms.
This only worked before because DEX2OAT_XMS, etc. were blank
due to bugs in build/, which resulted in --runtime-arg --runtime-arg
and anything starting with -- as the argument to --runtime-arg is
silently ignored.

Change-Id: I18607a6fd8d0004466404045f8dc4495eb6ab275
diff --git a/Android.mk b/Android.mk
index 3f4ead6..7a95dfe 100644
--- a/Android.mk
+++ b/Android.mk
@@ -324,7 +324,7 @@
 
 $$(OUT_OAT_FILE): $(PRODUCT_OUT)/$(1) $(DEFAULT_DEX_PREOPT_BUILT_IMAGE) $(DEX2OATD_DEPENDENCY)
 	@mkdir -p $$(dir $$@)
-	$(DEX2OATD) --runtime-arg $(DEX2OAT_XMS) --runtime-arg $(DEX2OAT_XMX) \
+	$(DEX2OATD) --runtime-arg -Xms$(DEX2OAT_XMS) --runtime-arg -Xmx$(DEX2OAT_XMX) \
 		--boot-image=$(DEFAULT_DEX_PREOPT_BUILT_IMAGE) --dex-file=$(PRODUCT_OUT)/$(1) \
 		--dex-location=/$(1) --oat-file=$$@ \
 		--instruction-set=$(DEX2OAT_TARGET_ARCH) \