Android: Create AVD dialog, give messagebox the right parent

Change-Id: Ib4f65eb0b63fdcb03468d28bf6c65a50074182bb
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Android: Create AVD: Show error messages

The check for errors was wrong and didn't even get the right string.
Fix that, and also use a QMessageBox for the errors.

Task-number: QTCREATORBUG-11729

Change-Id: I9aecd1dcbeabb35d60833419f7997ea4c704d65b
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>