title: getUidTxBytes(int uid) always return 0 in android 6.0
body: <p>I am trying to get network traffic stats of all apps. I just print total network traffic of every application in my device. The code is working fine in android 4.4 and 5.1 devices but in android 6.0 device it always return 0 for all applications. Anyone can please tell me why this happened in android 6.0 devices.</p>
  
  <pre><code>protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
  
      for(ApplicationInfo app : getPackageManager().getInstalledApplications(0)){
          long tx = TrafficStats.getUidTxBytes(app.uid);
          long rx = TrafficStats.getUidRxBytes(app.uid);
          long total = tx + rx;
          Log.e("total data of ", app.packageName + " = " + total);
      }
  }
  </code></pre>
  
  <p>Here's my <code>AndroidManifest.xml</code></p>
  
  <pre><code>&lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.mts.trafficstatsdemo"&gt;
  
  &lt;uses-permission android:name="android.permission.INTERNET"/&gt;
  &lt;uses-permission android:name="android.permission.READ_PHONE_STATE"/&gt;
  
  &lt;application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:supportsRtl="true"
      android:theme="@style/AppTheme"&gt;
      &lt;activity android:name=".MainActivity"&gt;
          &lt;intent-filter&gt;
              &lt;action android:name="android.intent.action.MAIN" /&gt;
  
              &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
          &lt;/intent-filter&gt;
      &lt;/activity&gt;
  &lt;/application&gt;
  </code></pre>