Add tracker to NotificationProgressBar.

The available length for the tracker to move is:
   available = trackWidth (minus padding) - trackerWidth.
The tracker moves evenly by scale, from scale = 0 to 1. See
`setTrackerPos` for the implementation.

Flag: android.app.api_rich_ongoing
Fix: 367805202
Fix: 372908099
Bug: 372909308
Test: post a ProgressStyle Notification
Test: patch ag/30006048 and run ProgressStyleNotificationScreenshotTest
Test: atest NotificationProgressBarTest
Change-Id: Iae744bea726db8f9f9bc029b17eef5933e909d44