[autotest] bluetooth_device_xmlrpc_server: device_is_connected method

Add an API to query if a device is connected.

BUG=chromium:545808
TEST=not used yet

Change-Id: I866aa0efc9801939a8e7ca635564a6768a97efd0
Reviewed-on: https://chromium-review.googlesource.com/307642
Commit-Ready: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
diff --git a/server/cros/bluetooth/bluetooth_device.py b/server/cros/bluetooth/bluetooth_device.py
index 033b9d2..86166d5 100644
--- a/server/cros/bluetooth/bluetooth_device.py
+++ b/server/cros/bluetooth/bluetooth_device.py
@@ -276,6 +276,17 @@
         return self._proxy.connect_device(address)
 
 
+    def device_is_connected(self, address):
+        """Checks if a device is connected.
+
+        @param address: Address of the device to check if it is connected.
+
+        @returns: True if device is connected. False otherwise.
+
+        """
+        return self._proxy.device_is_connected(address)
+
+
     def disconnect_device(self, address):
         """Disconnects a device.