要实现在应用程序前台使用不同的图标，而在应用程序后台使用另一个图标，可以使用以下步骤：

1. 首先，在res目录下创建两个不同的图标资源文件，例如：icon_foreground.png和icon_background.png。
2. 在AndroidManifest.xml文件中的应用程序标签中添加以下属性：

这将设置应用程序的前台图标为icon_foreground.png。

3. 在应用程序的主要活动类中添加以下代码来监听应用程序的前后台状态：

public class MainActivity extends AppCompatActivity {

    private boolean isAppInForeground;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 判断应用程序是否在前台
        isAppInForeground = true;
        registerActivityLifecycleCallbacks(new Application.ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(@NonNull Activity activity, @Nullable Bundle savedInstanceState) {

            }

            @Override
            public void onActivityStarted(@NonNull Activity activity) {

            }

            @Override
            public void onActivityResumed(@NonNull Activity activity) {
                isAppInForeground = true;
                updateAppIcon();
            }

            @Override
            public void onActivityPaused(@NonNull Activity activity) {
                isAppInForeground = false;
                updateAppIcon();
            }

            @Override
            public void onActivityStopped(@NonNull Activity activity) {

            }

            @Override
            public void onActivitySaveInstanceState(@NonNull Activity activity, @NonNull Bundle outState) {

            }

            @Override
            public void onActivityDestroyed(@NonNull Activity activity) {

            }
        });
    }

    private void updateAppIcon() {
        if (isAppInForeground) {
            setAppIcon(R.mipmap.icon_foreground);
        } else {
            setAppIcon(R.mipmap.icon_background);
        }
    }

    private void setAppIcon(int resId) {
        try {
            ActivityInfo activityInfo = getPackageManager().getActivityInfo(getComponentName(), PackageManager.GET_META_DATA);
            Bundle metaData = activityInfo.metaData;
            String appIcon = metaData.getString("com.google.android.gms.wallet.api.enabled");

            if (appIcon != null && appIcon.equals("android")) {
                int iconRes = getResources().getIdentifier("com.google.android.gms.wallet.api.enabled", "drawable", getPackageName());
                if (iconRes != 0) {
                    resId = iconRes;
                }
            }
        } catch (PackageManager.NameNotFoundException e) {
            e.printStackTrace();
        }

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (getApplicationContext().getPackageManager().canRequestPackageInstalls()) {
                updateAppIcon(resId);
            } else {
                Intent intent = new Intent(Settings.ACTION_MANAGE_UNKNOWN_APP_SOURCES, Uri.parse("package:" + getApplicationContext().getPackageName()));
                startActivityForResult(intent, REQUEST_INSTALL_UNKNOWN_APP);
            }
        } else {
            updateAppIcon(resId);
        }
    }

    private void updateAppIcon(int resId) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N_MR1) {
            ShortcutManager shortcutManager = getSystemService(ShortcutManager.class);
            if (shortcutManager.getDynamicShortcuts().size() > 0) {
                ShortcutInfo shortcutInfo = shortcutManager.getDynamicShortcuts().get(0);
                ShortcutInfo.Builder builder = new ShortcutInfo.Builder(this, shortcutInfo.getId());
                Icon icon = Icon.createWithResource(this, resId);
                builder.setIcon(icon);
                shortcutManager.updateShortcuts(Arrays.asList(builder.build()));
            }
        } else {
            Intent intent = new Intent(Intent.ACTION_MAIN);
            intent.setClass(getApplicationContext(), MainActivity.class);
            intent.addCategory(Intent.CATEGORY_LAUNCHER);
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);
            PendingIntent pendingIntent = PendingIntent.getActivity(getApplicationContext(), 0, intent, 0);
            try {
                Method method = NotificationManager.class.getMethod("setIconBadgeNum", int.class, int.class, PendingIntent.class);
                method.invoke(null, getApplicationContext().getPackageManager().getApplicationInfo(getPackageName(), 0).uid, resId, pendingIntent);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

这段代码将监听应用程序的生命周期，并在应用程序进入前台或后台时更新应用图标。

4. 在AndroidManifest.xml文件中的标签中添加以下元数据：