[Android] Output unchecked warnings and fix existing instances.

build/android/ant/sdk-targets.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <!--
   Copyright (c) 2012 The Chromium Authors. All rights reserved.
   Use of this source code is governed by a BSD-style license that can be
   found in the LICENSE file.
 -->
 <project name="chrome_sdk_overrides" >
   <!--
     Redefinition of targets used by SDK tools.
     Supported version: SDK tools revision 20.
@@ skipping 44 matching lines @@
           fork="${need.javac.fork}"
           includeantruntime="false"
           source="${java.source}"
           target="${java.target}"
           verbose="${verbose}">
         <src path="${source.absolute.dir}"/>
         <src path="${gen.absolute.dir}"/>
         <src>
           <path refid="javac.srcdirs.additional"/>
         </src>
+        <compilerarg value="-Xlint:unchecked"/>
         <compilerarg line="${java.compilerargs}"/>
       </javac>
       <!--
         If the project is instrumented, then instrument the classes
         TODO(shashishekhar): Add option to override emma filter.
       -->
       <if condition="${build.is.instrumented}">
         <then>
           <echo level="info">
             Instrumenting classes from ${out.absolute.dir}/classes...
@@ skipping 193 matching lines @@
           -libraryjars  ${project.target.classpath.value}
           -dump         "${obfuscate.absolute.dir}/dump.txt"
           -printseeds   "${obfuscate.absolute.dir}/seeds.txt"
           -printusage   "${obfuscate.absolute.dir}/usage.txt"
           -printmapping "${obfuscate.absolute.dir}/mapping.txt"
         </proguard>
       </then>
     </if>
   </target>
 </project>