Android : Disable button which shows me all running apps

Normally, as we see in most of the Android devices, long presses home button shows us all running applications, but in my tab i have separate button for that which supports Android 4.4.2 version, see button within a circle and Email app which i am currently running on my tab in below image:

Now i would like to disable button, which shows me all running apps on my device !

In my activity i already used below code to disable back button functionality:

@Override
public void onBackPressed() {

}