refactor(core): introduce APP_BOOTSTRAP_LISTENER multi provider

Using the `registerBootstrapListener` easily lead to race condition
and needed dependencies on `ApplicationRef`.

BREAKING CHANGE:
- `ApplicationRef.registerBootstrapListener` is deprecated. Provide a multi
  provider for the new token `APP_BOOTSTRAP_LISTENER` instead.

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

@@ -53,6 +53,13 @@ export const PLATFORM_INITIALIZER: any = new OpaqueToken('Platform Initializer')
  */
 export const APP_INITIALIZER: any = new OpaqueToken('Application Initializer');
 
+/**
+ * All callbacks provided via this token will be called when a component has been bootstrapped.
+ *
+ * @experimental
+ */
+export const APP_BOOTSTRAP_LISTENER = new OpaqueToken('appBootstrapListener');
+
 /**
  * A token which indicates the root directory of the application
  * @experimental