mojo_run: support parallel Android runs of mojo_run.

mojo/devtools/common/devtoolslib/android_shell.py

Index: mojo/devtools/common/devtoolslib/android_shell.py
diff --git a/mojo/devtools/common/devtoolslib/android_shell.py b/mojo/devtools/common/devtoolslib/android_shell.py
index 275a3dce94a49eabf809397af38ee4257b0841ce..611732e045da426e1836253a76d3c78f038f30a8 100644
--- a/mojo/devtools/common/devtoolslib/android_shell.py
+++ b/mojo/devtools/common/devtoolslib/android_shell.py
@@ -408,18 +408,20 @@ class AndroidShell(Shell):
     logcat_watch_thread.start()
 
   @overrides(Shell)
-  def serve_local_directory(self, local_dir_path, port=0):
+  def serve_local_directory(self, local_dir_path, port=0, free_host_port=False):
     assert local_dir_path
     mappings = [('', [local_dir_path])]
-    server_address = start_http_server(mappings, host_port=port)
+    host_port = 0 if free_host_port else port
+    server_address = start_http_server(mappings, host_port=host_port)
 
     return 'http://127.0.0.1:%d/' % self._forward_device_port_to_host(
         port, server_address[1])
 
   @overrides(Shell)
-  def serve_local_directories(self, mappings, port=0):
+  def serve_local_directories(self, mappings, port=0, free_host_port=False):
     assert mappings
-    server_address = start_http_server(mappings, host_port=port)
+    host_port = 0 if free_host_port else port
+    server_address = start_http_server(mappings, host_port=host_port)
 
     return 'http://127.0.0.1:%d/' % self._forward_device_port_to_host(
         port, server_address[1])