要在显示插播广告之前添加计时器，您可以使用Android中的Handler对象延迟广告的显示。以下是如何修改代码的示例：

private void goMainScreen(int position) {
    if (mInterstitialAd != null) {
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                mInterstitialAd.show(MainActivity.this);
            }
        }, 5000); // 5000 milliseconds = 5 seconds

        mInterstitialAd.setFullScreenContentCallback(new FullScreenContentCallback() {
            @Override
            public void onAdClicked() {
                super.onAdClicked();
            }

            @Override
            public void onAdDismissedFullScreenContent() {
                super.onAdDismissedFullScreenContent();
                startActivity(new Intent(MainActivity.this, SignsActivity.class));
                Pref.getInstance(MainActivity.this).setIndex(position);
            }

            @Override
            public void onAdFailedToShowFullScreenContent(@NonNull AdError adError) {
                super.onAdFailedToShowFullScreenContent(adError);
                startActivity(new Intent(MainActivity.this, SignsActivity.class));
                Pref.getInstance(MainActivity.this).setIndex(position);
            }

            @Override
            public void onAdImpression() {
                super.onAdImpression();
            }

            @Override
            public void onAdShowedFullScreenContent() {
                super.onAdShowedFullScreenContent();
            }
        });

    } else {
        startActivity(new Intent(MainActivity.this, SignsActivity.class).putExtra("index", position));
        Pref.getInstance(MainActivity.this).setIndex(position);
    }

    adPosition = adPosition + 1;
}

在上面的代码中，我们使用Handler类的postDelayed方法将插入式广告的显示延迟5000毫秒（5秒）。如果愿意，您可以将此值调整为不同的持续时间。