第一种方式

//得到栈顶Activity的名字，注意此处要进行判断，Android在5.0以后Google把getRunningTasks的方法给屏蔽掉了，所以要分开处理
    private static String getTopActivityName(Context context) {
        String topActivityPackageName;
        ActivityManager manager = (ActivityManager) context.getSystemService(context.ACTIVITY_SERVICE);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            //此处要判断用户的安全权限有没有打开，如果打开了就进行获取栈顶Activity的名字的方法
            //当然，我们的要求是如果没打开就不获取了，要不然跳转会影响用户的体验
            if (isSecurityPermissionOpen(context)) {
                UsageStatsManager mUsageStatsManager = (UsageStatsManager) context.getSystemService(Context.USAGE_STATS_SERVICE);
                long endTime = System.currentTimeMillis();
                long beginTime = endTime - 1000 * 60 * 2;
                UsageStats recentStats = null;

                List<UsageStats> queryUsageStats = mUsageStatsManager.queryUsageStats(UsageStatsManager.INTERVAL_BEST, beginTime, endTime);
                if (queryUsageStats == null || queryUsageStats.isEmpty()) {
                    return null;
                }

                for (UsageStats usageStats : queryUsageStats) {
                    if (recentStats == null || recentStats.getLastTimeUsed() < usageStats.getLastTimeUsed()) {
                        recentStats = usageStats;
                    }
                }
                topActivityPackageName = recentStats.getPackageName();
                return topActivityPackageName;
            } else {
               return null;
            }
        } else {
            List<ActivityManager.RunningTaskInfo> taskInfos = manager.getRunningTasks(1);
            if (taskInfos.size() > 0)
                topActivityPackageName = taskInfos.get(0).topActivity.getPackageName();
            else
                return null;
            return topActivityPackageName;
        }
    }

    //判断用户对应的安全权限有没有打开
    private static boolean isSecurityPermissionOpen(Context context) {
        long endTime = System.currentTimeMillis();
        UsageStatsManager usageStatsManager = (UsageStatsManager) context.getApplicationContext().getSystemService("usagestats");
        List<UsageStats> queryUsageStats = usageStatsManager.queryUsageStats(UsageStatsManager.INTERVAL_BEST, 0, endTime);
        if (queryUsageStats == null || queryUsageStats.isEmpty()) {
            return false;
        }
        return true;
    }

注意:此处是跳转安全权限的跳转代码，如果你判断用户没有开启权限的话可以选择跳转进行设置。
//Intent intent = new Intent(Settings.ACTION_USAGE_ACCESS_SETTINGS);
//startActivity(intent);

第二种方式

/**
     * 注意： getRunningAppProcesses()方法在5.0开始，就只返回自身应用的进程，所以只能判断自身进程状态，
     * 如果是400，返回为null,不能拿到当前栈顶Activity的包名
     *
     * @param context
     * @return
     */
    private static String getCurrentPkgName(Context context) {
        // 5x系统以后利用反射获取当前栈顶activity的包名.
        ActivityManager.RunningAppProcessInfo currentInfo = null;
        Field field = null;
        int startTaskToFront = 2;
        String pkgName = null;
        try {
            // 通过反射获取进程状态字段.
            field = ActivityManager.RunningAppProcessInfo.class.getDeclaredField("processState");
        } catch (Exception e) {
            e.printStackTrace();
        }
        ActivityManager am = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
        List appList = am.getRunningAppProcesses();
        ActivityManager.RunningAppProcessInfo app;
        for (int i = 0; i < appList.size(); i++) {
            //ActivityManager.RunningAppProcessInfo app : appList
            app = (ActivityManager.RunningAppProcessInfo) appList.get(i);
            //表示前台运行进程.
            if (app.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND) {
                Integer state = null;
                try {
                    // 反射调用字段值的方法,获取该进程的状态.
                    state = field.getInt(app);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                // 根据这个判断条件从前台中获取当前切换的进程对象
                if (state != null && state == startTaskToFront) {
                    currentInfo = app;
                    break;
                }
            }
        }
        if (currentInfo != null) {
            pkgName = currentInfo.processName;
        }
        return pkgName;
    }