[Android] Output unchecked warnings and fix existing instances.

build/android/ant/chromium-jars.xml

 <!--
   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="chromium-jars" default="dist">
   <!--
     Common ant build file for for chromium_*.jars.
     For creating a new chromium_*.jar :
     1. Use build/java.gypi action.
@@ skipping 51 matching lines @@
 
   <target name="compile" depends="init" description="Compiles source.">
     <fail message="Error: javac.custom.classpath is not set. Please set it to
         classpath for javac.">
       <condition>
         <not><isreference refid="javac.custom.classpath"/></not>
       </condition>
     </fail>
 
     <javac
-      srcdir="${javac.srcdir}"
-      destdir="${dest.dir}"
-      classpathref="javac.custom.classpath"
-      debug="true"
-      includeantruntime="false"
-      includes="${JAVAC_INCLUDES}"
-    />
+        srcdir="${javac.srcdir}"
+        destdir="${dest.dir}"
+        classpathref="javac.custom.classpath"
+        debug="true"
+        includeantruntime="false"
+        includes="${JAVAC_INCLUDES}">
+      <compilerarg value="-Xlint:unchecked"/>
+    </javac>
   </target>
 
   <target name="dist" depends="compile"
       description="Generate chromium_${PACKAGE_NAME}.jar.">
     <!-- Create the distribution directory.  We exclude R.class and R$*.class
          files since new versions of these files with the correct resource -> ID
          mapping will be provided when we build each individual apk. -->
     <jar
       jarfile="${lib.java.dir}/chromium_${PACKAGE_NAME}.jar"
       excludes="**/R.class **/R$*.class"
       basedir="${dest.dir}"
     />
 
     <!-- If Gyp thinks this output is stale but Ant doesn't, the modification
          time should still be updated. Otherwise, this target will continue to
          be rebuilt in future builds.
     -->
     <touch file="${lib.java.dir}/chromium_${PACKAGE_NAME}.jar"/>
   </target>
 
   <target name="clean" description="clean up">
     <!-- Delete the appropriate directory trees -->
     <delete dir="${dest.dir}"/>
   </target>
 </project>