Fix `mojo_run --android` to terminate when the shell terminates.

mojo/devtools/common/devtoolslib/android_shell.py

 # Copyright 2014 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.
 
 import atexit
 import hashlib
 import json
 import logging
 import os
 import os.path
@@ skipping 348 matching lines @@
         match = re.search(r'Observatory listening on http://127.0.0.1:(\d+)',
                           line)
         if match:
           device_port = int(match.group(1))
           host_port = self._forward_host_port_to_device(0, device_port)
           print ("Dart observatory available at the host at http://127.0.0.1:%d"
                  % host_port)
 
     logcat_watch_thread = threading.Thread(
         target=_forward_observatories_as_needed)
+    logcat_watch_thread.daemon = True
     logcat_watch_thread.start()
 
   @overrides(Shell)
   def serve_local_directory(self, local_dir_path, port=0):
     assert local_dir_path
     mappings = [('', [local_dir_path])]
     server_address = start_http_server(mappings, host_port=port)
 
     return 'http://127.0.0.1:%d/' % self._forward_device_port_to_host(
         port, server_address[1])
@@ skipping 38 matching lines @@
           Results.output = rf.read()
 
     run_thread = threading.Thread(target=do_run)
     run_thread.start()
     run_thread.join(timeout)
 
     if run_thread.is_alive():
       self.stop_shell()
       return None, Results.output, True
     return None, Results.output, False