file_poller: fixed validation with Linux devices

- device.is_rooted check was being perfromed inside validate() which
  gets invoked before the device is connected. This worked for most
  Android devices, because connections is a no-op for them, however
  failed for Linux targets. The check is now performed inside
  initialize().
- Added _is_ready() check inside is_rooted to catch similar problems
  quicker in the future.

wlauto/common/linux/device.py

@@ -598,6 +598,7 @@ class LinuxDevice(BaseLinuxDevice):
 
     @property
     def is_rooted(self):
+        self._check_ready()
         if self._is_rooted is None:
             # First check if the user is root
             try: