Add option to exclude SystemUI tests

Setting the environment variable EXCLUDE_SYSTEMUI_TESTS to something
excludes the tests package during building. Saves a few seconds for
incremental changes in SystemUI for fast UI iteration changes.

Change-Id: I596a0273133c8a8bece0cc364e2ec61fdea45116
diff --git a/packages/SystemUI/Android.mk b/packages/SystemUI/Android.mk
index 69c6159..2ff9a28 100644
--- a/packages/SystemUI/Android.mk
+++ b/packages/SystemUI/Android.mk
@@ -22,4 +22,6 @@
 
 include $(BUILD_PACKAGE)
 
-include $(call all-makefiles-under,$(LOCAL_PATH))
+ifeq ($(EXCLUDE_SYSTEMUI_TESTS),)
+    include $(call all-makefiles-under,$(LOCAL_PATH))
+endif