feat(drag-drop): make `CDK_DROP_CONTAINER` token public api

[jorroll]

Bug, feature request, or proposal:

feature request

What is the expected behavior?

That the CDK_DROP_CONTAINER injection token be public API.

What is the current behavior?

The CDK_DROP_CONTAINER is private api.

What are the steps to reproduce?

See injection token located in cdk-experimental/drag-drop here:
https://github.com/angular/material2/blob/master/src/cdk-experimental/drag-drop/drop-container.ts#L58

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

Making the CDK_DROP_CONTAINER token public API is necessary in order to support creating a custom drop component which extends CdkDrop. The custom drop component will need to re-provide CDK_DROP_CONTAINER.

i.e

@Component({
  providers: [
    {provide: CDK_DROP_CONTAINER, useExisting: MyCustomListComponent},
  ],
})
export class MyCustomListComponent extends CdkDrop<ListRowItem> {}

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

I'm using

- @angular/core 6.0.7
- @angular/material 6.3.2
- @angular/cdk 6.3.2
- @angular/cdk-experimental 6.3.3

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