feat(animations): support styling of the default animation state

It is now possible to set a fallback state that will apply its
styling when the destination state is not detected.

```ts
state("*", style({ ... }))
```

Closes #9013

modules/@angular/core/src/animation/animation_constants.ts

@@ -1,3 +1,4 @@
 export const FILL_STYLE_FLAG = 'true'; // TODO (matsko): change to boolean
 export const ANY_STATE = '*';
+export const DEFAULT_STATE = '*';
 export const EMPTY_STATE = 'void';

modules/@angular/core/src/animation/metadata.ts

@@ -292,6 +292,10 @@ export function style(tokens: string|{[key: string]: string | number}|Array<stri
  * of the application anymore (e.g. when an `ngIf` evaluates to false then the state of the associated element
  * is void).
  *
+ * #### The `*` (default) state
+ *
+ * The `*` state (when styled) is a fallback state that will be used if
+ * the state that is being animated is not declared within the trigger.
  *
  * ### Usage
  *