<uses-permission android:name="android.permission.PACKAGE_USAGE_STATS" />

String topPackageName ;

if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {

    UsageStatsManager mUsageStatsManager = (UsageStatsManager)getSystemService("usagestats");

    long time = System.currentTimeMillis();

    // We get usage stats for the last 10 seconds

    List<UsageStats> stats = mUsageStatsManager.queryUsageStats(UsageStatsManager.INTERVAL_DAILY, time - 1000*10, time);

    // Sort the stats by the last time used

    if(stats != null) {

        SortedMap<Long,UsageStats> mySortedMap = new TreeMap<Long,UsageStats>();

        for (UsageStats usageStats : stats) {

            mySortedMap.put(usageStats.getLastTimeUsed(),usageStats);

        }

        if(mySortedMap != null && !mySortedMap.isEmpty()) {

            topPackageName =  mySortedMap.get(mySortedMap.lastKey()).getPackageName();

        }

    }

}