valid dd/mm format switch sides after manual input on datepicker field

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
That the valid `dd/mm/yyyy` inputed date remains as it is, not matter what, they always switch sides on change.

#### What is the current behavior?
E.g.: `03/02/1990` becomes `02/03/1990`.

#### What are the steps to reproduce?
Set a custom locale like Brasil:

```typescript
// location
import localePt from '@angular/common/locales/pt';
import localePtExtra from '@angular/common/locales/extra/pt';

registerLocaleData(localePt, 'pt', localePtExtra);
```

Then try to use the datepicker with manual input.

#### What is the use-case or motivation for changing an existing behavior?
Because it works very well with the Angular locale config, there's just this switch bug that does not work properly.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
```json
{
"@angular/core": "^6.0.6",
"@angular/material": "^6.3.0",
}
```

#### Is there anything else we should know?

##### UI input:


##### Manual input:

~I really don't wanna use moment.js~

----

It seems like the StackBlitz example isn't wroking as well: https://stackblitz.com/angular/vnvbdjykmve?file=app%2Fdatepicker-locale-example.ts

Commencez par l’exemple StackBlitz lié dans app/datepicker-locale-example.ts et reproduisez le comportement de saisie manuelle du datepicker après avoir enregistré la locale pt et les données de locale supplémentaires. Suivez l’analyse syntaxique dépendante de la locale du datepicker et la gestion des changements ; le travail est terminé lorsqu’une valeur saisie telle que 03/02/1990 reste identique après la modification du champ.

Rédigé par le modèle d'indexation à partir du texte de l'issue.