Add explicitly merge_zips to otatools package

Since it's under soong/out/host, we need to explicitly copy it into the
otatools.zip

Bug: 146508800
Test: build and check otatools.zip
Change-Id: I83e201601cf03b6eb01ead912a78f22f750aa4be
diff --git a/core/Makefile b/core/Makefile
index a8c5be3..6ccc1ff 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -3892,7 +3892,7 @@
 	mkdir -p $(dir $@)
 	$(call copy-files-with-structure,$(PRIVATE_OTATOOLS_PACKAGE_FILES),$(HOST_OUT)/,$(PRIVATE_ZIP_ROOT))
 	$(call copy-files-with-structure,$(PRIVATE_OTATOOLS_RELEASETOOLS),build/make/tools/,$(PRIVATE_ZIP_ROOT))
-	cp $(SOONG_ZIP) $(ZIP2ZIP) $(PRIVATE_ZIP_ROOT)/bin/
+	cp $(SOONG_ZIP) $(ZIP2ZIP) $(MERGE_ZIPS) $(PRIVATE_ZIP_ROOT)/bin/
 	$(SOONG_ZIP) -o $@ -C $(PRIVATE_ZIP_ROOT) -D $(PRIVATE_ZIP_ROOT)
 
 .PHONY: otatools-package