Use iPart to identify points at start/end to avoid rounding shift.

Test notification config, on Felix:
  - 10 segments, each of length 10
  - 4 points, at 0, 1, 55, 100

Flag: android.app.api_rich_ongoing
Bug: 372908709
Test: Post ProgressStyle notifs via test app
Test: NotificationProgressBarTest still passes
Change-Id: Ia3eae382446cedb0cd8531a046dc2dd91aacfc9a