Bazel: Increase severity of ImmutableAnnotationChecker to ERROR

Change-Id: I31b03a0b8daeeda52fddc0a4278c1d902bd19c67

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index 6d15a21..19db90b 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -44,7 +44,7 @@
         "-Xep:FunctionalInterfaceClash:ERROR",
         "-Xep:FutureReturnValueIgnored:ERROR",
         "-Xep:GetClassOnEnum:ERROR",
-        "-Xep:ImmutableAnnotationChecker:WARN",
+        "-Xep:ImmutableAnnotationChecker:ERROR",
         "-Xep:ImmutableEnumChecker:WARN",
         "-Xep:IncompatibleModifiers:ERROR",
         "-Xep:InjectOnConstructorOfAbstractClass:ERROR",