Autocomplete should always handle display value and model separately

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

Feature request / proposal

#### What is the expected behavior?

Assuming the following:

**TS**

```typescript
options = [{name: 'One'}, {name: 'Two'}];
text: string;
```

**HTML**

```html

{{option.name}}

```

I expect `text` to be of type `string` at any time. When selecting an option from the dropdown, I do not expect that value to be assigned to the input value. MatAutocomplete should provide a separate `ngModel` to be bound to. Sure, the selected option should be displayed in the input, but using a `string`. This is possible via the `[displayWith]` input of ``, but should not be an "opt-in" feature, it should be required.

#### What is the current behavior?

When you select an option, `text` is now of type `{name: string}`, because Angular Material autocomplete assigns the selected value to it.

#### What are the steps to reproduce?

#### What is the use-case or motivation for changing an existing behavior?

When you fetch search results from a backend, you're usually subscribing to `valueChanges` (if using `FormControl`) or `ngModelChange` (if using `ngModel`). You would expect such subscriptions to stream values of the same type at runtime.

In [the docs](
https://material.angular.io/components/autocomplete/overview#setting-separate-control-and-display-values), you have worked around this problem by doing the following:

```
map(value => typeof value === 'string' ? value : value.name),
```

Which works fine, but is not intuitive, and semantically awkward. The docs aren't clear about this specific semantic inconsistency which may cause users (like me) to waste some time trying to understand why typing is not preserved until realizing it's not a bug, but an intended behavior.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular 7

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

Start with the autocomplete overview's “setting separate control and display values” section and the mat-autocomplete entry point, including displayWith and ngModel behavior. Clarify the proposed API and its compatibility with Angular 7 before changing anything; done should mean the selected option is displayed as a string while the bound text value remains a string, with the documented behavior made intuitive.

Written by the indexing model from the issue text.