Virtual scroll collectionViewer.viewChange does not emit when initial value was provided

#### Reproduction

Use StackBlitz to reproduce your issue: https://stackblitz.com/edit/angular-lakoup

Steps to reproduce:
1. Open stackblitz
2. Monitor the console and confirm that `collectionViewer.viewChange`does not emit


#### Expected Behavior

I am expecting `collectionViewer.viewChange`to fire

#### Actual Behavior

It does not fire, if the `_dataStream` has an initial value

#### Environment

- Angular: 8.1.4
- CDK/Material: 8.1.4
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): macOS

Start with the linked StackBlitz reproduction and inspect the virtual-scroll path involving collectionViewer.viewChange and the _dataStream initial value. Confirm the missing emission in the console, then trace the relevant CDK source and tests. Done means viewChange emits when the data stream already has an initial value, with regression coverage for that case.

Written by the indexing model from the issue text.