title: mapping.txt file is not getting generated when using Proguard
body: <p>I am facing issue while obfuscating my app using proguard.</p>
  
  <p>I am able to use proguard and it is generating mapping.txt when i am creating a new application in android studio, however mapping.txt is not being generated when i am trying to use proguard on a project migrated from eclipse to android studio.</p>
  
  <p>Following is my proguard-rules.pro file</p>
  
  <pre><code>-dontobfuscate
  -dontshrink
  -keepdirectories
  -keeppackagenames javax.mail.**
  -keeppackagenames javax.activation.**
  -keeppackagenames com.sun.mail.**
  -keeppackagenames myjava.**
  -keeppackagenames org.apache.harmony.**
  -keeppackagenames mailcap.**
  -keeppackagenames mimetypes.**
  -keep class javamail.** {*;}
  -keep class javax.mail.** {*;}
  -keep class javax.activation.** {*;}
  -keep class javax.mail.internet.** {*;}
  
  -keep class com.sun.mail.dsn.** {*;}
  -keep class com.sun.mail.handlers.** {*;}
  -keep class com.sun.mail.smtp.** {*;}
  -keep class com.sun.mail.util.** {*;}
  
  -keep class javax.ws.rs.** { *; }
  
  -keep class mailcap.** {*;}
  -keep class mimetypes.** {*;}
  -keep class myjava.awt.datatransfer.** {*;}
  -keep class org.apache.harmony.awt.** {*;}
  -keep class org.apache.harmony.misc.** {*;}
  
  -dontwarn com.fasterxml.jackson.**
  -dontwarn java.awt.**
  -dontwarn java.beans.Beans
  -dontwarn javax.security.**
  
  -ignorewarnings
  -printmapping build/outputs/mapping/release/mapping.txt
  </code></pre>