Returns:
            - lock state(bool)
        Raises:
            RuntimeError

def is_locked(self):
        """
        Returns:
            - lock state(bool)
        Raises:
            RuntimeError
        """
        _lockScreenRE = re.compile('mShowingLockscreen=(true|false)')
        m = _lockScreenRE.search(self.shell('dumpsys', 'window', 'policy'))
        if m:
            return (m.group(1) == 'true')
        raise RuntimeError("Couldn't determine screen lock state")