[Activity(Label = "App1", MainLauncher = true, Icon = "@drawable/icon",
    Theme = "@android:style/Theme.Light.NoTitleBar.Fullscreen")]
[IntentFilter(new[] { Intent.ActionMain }, Categories = new[] {
    Intent.CategoryHome, Intent.CategoryDefault })]

public override void OnBackPressed(){}

Java.Lang.Process proc = Java.Lang.Runtime.GetRuntime().Exec(
    new String[] { "su", "-c", "service call activity 42 s16 com.android.systemui" });
proc.Wait();

private void hideBar(){
try
{
    Process proc = Runtime.getRuntime().exec(new String[]{"su","-c","service call activity 42 s16 com.android.systemui"});
    proc.waitFor();
}
catch(Exception ex)
{
    Toast.makeText(getApplicationContext(), ex.getMessage(), Toast.LENGTH_LONG).show();
    Log.e("ROOT ERROR", ex.getMessage());
}

adb shell
su
pm disable com.android.systemui //(enable) чтобы включить обратно

@Override
  public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
      return true;
    }
    return false;
  }

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    getWindow().addFlags(WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY);

    setContentView(R.layout.activity_main);

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