Sync process movement with tracker via effectively padding the bar.

1. Start the bar at trackerDrawWidth/2 instead of 0; and end at
   w-trackerDrawWidth/2.
2. Drop the points at start/end, so the bar always starts and ends with
   a segment.
   - ProgressStyle already supports start/end icons outside the bar.
     Therefore no use case for start/end points any way.
3. Extend first and last segment to visually cover the
   trackerDrawWidth/2 spaces.

Cleaned up special handling logic for start/end points.

Also renamed mTrackerWidth to mTrackerDrawWidth to better convey its
usage and improve readability.

Flag: android.app.api_rich_ongoing
Bug: 391678201
Test: Post ProgressStyle notifications via test app
Test: patch ag/30006048 and run ProgressStyleNotificationScreenshotTest on device
Test: NotificationProgressBarTest
Change-Id: Ib72d2cd0685a471a86c0f97a21a60418d13ba788