[Android checkstyle] Allow underscores in package names.

tools/android/checkstyle/chromium-style-5.0.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE module PUBLIC "-//Puppy Crawl//DTD Check Configuration 1.3//EN" "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
 
 <!--
   See installation instructions: https://sites.google.com/a/chromium.org/dev/checkstyle
 -->
 <module name="Checker">
   <property name="severity" value="warning"/>
   <property name="charset" value="UTF-8"/>
   <module name="TreeWalker">
@@ skipping 20 matching lines @@
     </module>
     <module name="JavadocMethod">
       <property name="severity" value="warning"/>
       <property name="scope" value="public"/>
       <property name="allowMissingParamTags" value="true"/>
       <property name="allowMissingPropertyJavadoc" value="true"/>
       <property name="allowMissingReturnTag" value="true"/>
       <property name="allowMissingThrowsTags" value="true"/>
     </module>
     <module name="PackageName">
-      <property name="severity" value="warning"/>
-      <property name="format" value="^[a-z]+(\.[a-z][a-z0-9]{1,})*$"/>
+      <property name="severity" value="error"/>
+      <property name="format" value="^[a-z]+(\.[a-z][a-z0-9_]{1,})*$"/>
     </module>
     <module name="TypeName">
       <property name="severity" value="warning"/>
     </module>
     <module name="ConstantName">
       <property name="severity" value="error"/>
       <property name="format" value="^([A-Z][A-Z0-9]*(_[A-Z0-9]+)*)|(s[A-Z][a-zA-Z0-9]*)$"/>
       <message key="name.invalidPattern" value="Static final field names must either be all caps (e.g. int HEIGHT_PX) for 'true' constants, or start with s (e.g. AtomicInteger sNextId or Runnable sSuspendTask) for fields with mutable state or that don't 'feel' like constants."/>
     </module>
     <!-- Non-public, non-static field names start with m. -->
@@ skipping 98 matching lines @@
   <module name="RegexpSingleline">
     <property name="severity" value="error"/>
     <property name="format" value="[ \t]+$"/>
     <property name="message" value="Trailing whitespace"/>
     </module>
   <module name="RegexpHeader">
     <property name="severity" value="error"/>
     <property name="header" value="^// Copyright 20\d\d The Chromium Authors. All rights reserved.$\n^// Use of this source code is governed by a BSD-style license that can be$\n^// found in the LICENSE file.$"/>
   </module>
 </module>