Support mobile method 'launchApp' for selendroid

Hi

I used the method closeApp and launchApp to simulate restart the app when a test has failed before the next test start.
<code>
        """ restart the application """
        logger.info("Restart the app")
        self.driver.execute_script("mobile: closeApp")
        self.driver.execute_script("mobile: launchApp")
</code>