Bazel: Make 'DefaultCharset' an ERROR when compiling with ErrorProne toolchain

Change-Id: I6cc3b0323731a0ff4aa2d743938742ecc3900c74
diff --git a/tools/BUILD b/tools/BUILD
index 2571e75..cd34fe9 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -68,7 +68,7 @@
         "-Xep:CannotMockFinalClass:WARN",
         "-Xep:ClassCanBeStatic:WARN",
         "-Xep:ClassNewInstance:WARN",
-        "-Xep:DefaultCharset:WARN",
+        "-Xep:DefaultCharset:ERROR",
         "-Xep:DoubleCheckedLocking:WARN",
         "-Xep:ElementsCountedInLoop:WARN",
         "-Xep:EqualsHashCode:WARN",