How do I handle a back-button press in a (Delphi) Android app?

Here is an updated code for Remy's answer (works with Seattle):

procedure TForm1.FormKeyUp(Sender: TObject; var Key: Word; var KeyChar: Char; Shift: TShiftState);
var
  FService : IFMXVirtualKeyboardService;
begin
  if Key = vkHardwareBack then
  begin
    TPlatformServices.Current.SupportsPlatformService(IFMXVirtualKeyboardService, IInterface(FService));
    if (FService <> nil) and (TVirtualKeyboardState.Visible in FService.VirtualKeyBoardState) then
    begin
      // Back button pressed, keyboard visible, so do nothing...
    end else
    begin
      Key := 0;
      // Back button pressed, keyboard not visible or not supported on this platform, lets exit the app...
      MessageDlg('Exit Application?', TMsgDlgType.mtConfirmation, [TMsgDlgBtn.mbOK, TMsgDlgBtn.mbCancel], -1, OnCloseDialog);
    end;
  end;
end;

procedure TForm1.OnCloseDialog(Sender: TObject; const AResult: TModalResult);
begin
  if AResult = mrOK then
    Close;
end;

For future reference to anyone trying to make sense of this..

if Key = vkHardwareBack then
    begin
      // your code here
      key := 0;
end;

The key := 0; is the secret to stop the app from closing..

This goes in the forms OnKeyUp event

In the form's OnKey... events, the Key parameter is vkHardwareBack on Android. For example:

uses
  FMX.Platform, FMX.VirtualKeyboard;

procedure TForm1.FormKeyUp(Sender: TObject; var Key: Word; var KeyChar: Char; Shift: TShiftState);
var
  FService : IFMXVirtualKeyboardService;
begin
  if Key = vkHardwareBack then
  begin
    TPlatformServices.Current.SupportsPlatformService(IFMXVirtualKeyboardService, IInterface(FService));
    if (FService <> nil) and (vksVisible in FService.VirtualKeyBoardState) then
    begin
      // Back button pressed, keyboard visible, so do nothing...
    end else
    begin
      // Back button pressed, keyboard not visible or not supported on this platform, lets exit the app...
      if MessageDlg('Exit Application?', TMsgDlgType.mtConfirmation, [TMsgDlgBtn.mbOK, TMsgDlgBtn.mbCancel], -1) = mrOK then
      begin
        // Exit application here...
      end else
      begin
        // They changed their mind, so ignore the Back button press...
        Key := 0;
      end;
    end;
  end
  ...
end;

Going back to the previous screen depends on your application design.

• If you used TTabControl for displaying pages, you can navigate to the previous TTabItem.

• If you used TForms for displaying pages, you must use Close() procedure for closing the current form and going back to the previous screen.