[Android] Cleanup printed output. Showtask list

 This change includes the following:
  * Display task list executing
  * Combine redundant messages
  * Check system requirements up front
  * Show error from main ant package build process on error
  * Check for existence of 'template' directory before accessing
  * Capture KeyboardInterrupt exception to gracefully exit on Ctl+C

 Bug=XWALK-2392