refactor(core): Introduce AppInitStatus

This class allows any provider to know and wait for the initialization of the
application. This functionality previously was tied to `ApplicationRef`.

BREAKING CHANGE:
- `ApplicationRef.waitForAsyncInitializers` is deprecated. Use 
  `AppInitStatus.donePromise` / `AppInitStatus.done` instead.

modules/@angular/core/src/application_tokens.ts

@@ -47,12 +47,6 @@ function _randomChar(): string {
  */
 export const PLATFORM_INITIALIZER: any = new OpaqueToken('Platform Initializer');
 
-/**
- * A function that will be executed when an application is initialized.
- * @experimental
- */
-export const APP_INITIALIZER: any = new OpaqueToken('Application Initializer');
-
 /**
  * All callbacks provided via this token will be called when a component has been bootstrapped.
  *