In my case, the solution that worked was an improved version of @Hassnain Jamil's code:

Intent homeIntent = new Intent(Intent.ACTION_MAIN);
homeIntent.addCategory(Intent.CATEGORY_HOME);
homeIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
homeIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(homeIntent);
finishAffinity();
System.exit(0);