Remove a bunch of unused CTS tests from continuous test build to save space.

BUG 2053298
diff --git a/tools/cts-reference-app-lib/Android.mk b/tools/cts-reference-app-lib/Android.mk
index 6f64533..471f755 100644
--- a/tools/cts-reference-app-lib/Android.mk
+++ b/tools/cts-reference-app-lib/Android.mk
@@ -19,9 +19,10 @@
 
 LOCAL_SRC_FILES := $(call all-subdir-java-files)
 
+LOCAL_MODULE_TAGS := optional
+
 LOCAL_JAVA_LIBRARIES := android.test.runner
 
 LOCAL_MODULE := android.cts.refapp
-LOCAL_MODULE_TAGS := tests
 
 include $(BUILD_STATIC_JAVA_LIBRARY)