dgeni should export classes with @private as interfaces

This can be fixed along with #1594

ApplicationRef, for example, is in the d.ts correctly as an interface, but output to API docs as a class.

See: https://github.com/borisyankov/DefinitelyTyped/blob/3b0f362204ea965952dafdb682a5274d527ef9d0/angular2/angular2.d.ts#L2154

Essentially, need to make the same fix we did for the d.ts in the dgeni template.
https://github.com/angular/angular/issues/3002
