Also invoke "setProgressTrackerIcon" when mTrackerIcon is null.

This is necessary when the app is updating the same notification id. If
they want to remove the tracker icon for some reason, we should respect
their wish.

Flag: android.app.api_rich_ongoing
Bug: 379359830
Test: post a ProgressStyle notification with tracker icon, then set the
tracker icon to null and update the same notification id.

Change-Id: Iafa37cf6b920238d7ede49785785e34411242c41