Get Element By ID. How to get Resource ID?

Looks like you are testing on a webview. Just to make clear, webview ist he view in which u load an HTML page in Android.

In that case you need to change the context to webview before you call findBy method.

EX:

Check if view is webview -

            protected boolean isWebView() {
	boolean bWebView = false;
	android_driver.context("NATIVE_APP");
	try {
		Thread.sleep(3000);
	} catch (InterruptedException e) {
		// TODO Auto-generated catch block
		e.printStackTrace();
	}
	List<WebElement> mList = android_driver
			.findElementsByClassName("android.webkit.WebView");
	if (mList.size() > 0) {
		System.out.println("Webview available");
		android_driver.context("WEBVIEW_com.xxx.yyy.mobile");
		bWebView = true;
	} else {
		System.out.println("Webview NOT available");
		// android_driver.context("NATIVE_APP");
	}

	return bWebView;
}

Then you can call - MobileBy.xpath("/html/body/div[5]/div/div/div[1]/div[1]/div[3]/div[1]/div[2]/div[3]/div[2]")