Checks if the screen is ON.
        Returns:
            True if the device screen is ON
        Raises:
            RuntimeError

def is_screen_on(self):
        '''
        Checks if the screen is ON.
        Returns:
            True if the device screen is ON
        Raises:
            RuntimeError
        '''

        _screenOnRE = re.compile('mScreenOnFully=(true|false)')
        m = _screenOnRE.search(self.shell('dumpsys', 'window', 'policy'))
        if m:
            return (m.group(1) == 'true')
        raise RuntimeError("Couldn't determine screen ON state")