Stop application

        Args:
            package_name: string like com.example.app1
            clear: bool, remove user data

        Returns:
            None

def stop_app(self, package_name, clear=False):
        '''
        Stop application

        Args:
            package_name: string like com.example.app1
            clear: bool, remove user data

        Returns:
            None
        '''
        if clear:
            self.adb_shell(['pm', 'clear', package_name])
        else:
            self.adb_shell(['am', 'force-stop', package_name])
        return self