ErrorProne: Increase severity of FunctionalInterfaceClash to ERROR

Change-Id: I6008da8506eaa0764e8265a10384987ef0b1c3b1
diff --git a/tools/BUILD b/tools/BUILD
index 9a53c8b..fb3ed43 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -44,7 +44,7 @@
         "-Xep:FloatingPointLiteralPrecision:WARN",
         "-Xep:FragmentInjection:WARN",
         "-Xep:FragmentNotInstantiable:WARN",
-        "-Xep:FunctionalInterfaceClash:WARN",
+        "-Xep:FunctionalInterfaceClash:ERROR",
         "-Xep:FutureReturnValueIgnored:WARN",
         "-Xep:GetClassOnEnum:WARN",
         "-Xep:ImmutableAnnotationChecker:WARN",