获取android默认的useragent

兼容性最好的就是第一种，推荐使用，代码量少，简单易用

<pre>

方法一:

String useragent = new WebView(this).getSettings().getUserAgentString();

方法二:

private static String getUserAgent() {
    String userAgent = "";
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
        try {
            userAgent = WebSettings.getDefaultUserAgent(getInstance());
        } catch (Exception e) {
            userAgent = System.getProperty("http.agent");
        }
    } else {
        userAgent = System.getProperty("http.agent");
    }
    StringBuffer sb = new StringBuffer();
    for (int i = 0, length = userAgent.length(); i < length; i++) {
        char c = userAgent.charAt(i);
        if (c <= '\u001f' || c >= '\u007f') {
            sb.append(String.format("\\u%04x", (int) c));
        } else {
            sb.append(c);
        }
    }
    return sb.toString();
}

</pre>