servo: stop accessing _serialnames directly

Use get_serial_number and add_serial_number to interact with the
_serialnames dictionary.

BUG=none
BRANCH=none
TEST=start servod get all serial numbers

Change-Id: I37a5cdbb94aa13574222666ee767bf0e97c07284
Signed-off-by: Mary Ruthven <mruthven@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1700318
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
diff --git a/servo/servo_server.py b/servo/servo_server.py
index ebbf6ca..e4ed9bd 100755
--- a/servo/servo_server.py
+++ b/servo/servo_server.py
@@ -673,6 +673,17 @@
         rv.append(self.get(cmd))
     return rv
 
+  def add_serial_number(self, name, serial_number):
+    """Adds the serial number to the _serialnames dictionary.
+
+    Args:
+      name: A string which is the key into the _serialnames dictionary.
+      serial_number: A string which is the key into the _serialnames dictionary.
+    """
+    self._serialnames[name] = serial_number
+    self._logger.debug('Added %s %s to serialnames %r', name, serial_number,
+                       self._serialnames)
+
   def get_serial_number(self, name):
     """Returns the desired serial number from the serialnames dict.