Error out on unnecessary lambda (#4835)

codestyle/errorprone-rules.properties

@@ -1161,7 +1161,7 @@ URLEqualsHashCode=ERROR
 #UnnecessaryAssignment=WARN
 # Fields annotated with @Inject/@Mock should not be manually assigned to, as they should be initialized by a framework. Remove the assignment if a framework is being used, or the annotation if one isn't.
 
-#UnnecessaryLambda=WARN
+UnnecessaryLambda=ERROR
 # Returning a lambda from a helper method or saving it in a constant is unnecessary; prefer to implement the functional interface method directly and use a method reference instead.
 
 #UnnecessaryLongToIntConversion=WARN