To hide complete action bar use this code in MainActivity.java before calling this line SetContentview
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE); // This code will hide Action Bar form your UI
setContentView(R.layout.mailActivity);
Updated : use this to hide application icon
PackageManager p = getPackageManager();
ComponentName componentName = new ComponentName(this, com.apps.MainActivity.class); // activity which is first time open in manifiest file which is declare as <category android:name="android.intent.category.LAUNCHER" />
p.setComponentEnabledSetting(componentName,PackageManager.COMPONENT_ENABLED_STATE_DISABLED, PackageManager.DONT_KILL_APP);