Refactor to precalculate start/end positions for drawing Segment/Point.

The start/end positions for drawing will be precalculated and stored as
fields in NotifProgDrawable.Segment/Point, before they are
passed to NotifProgDrawable.setParts.

Also added documentation comment for `setProgress`, `setMin`, and
`setMax` to clarify that they only update the position of the tracker.

Also:
  - Cleaned up Point.mIcon, which was never used.
  - Cleaned up default color support for Segments and Points.

Flag: android.app.api_rich_ongoing
Bug: 372908709
Test: NotificationProgressBarTest
Test: Post ProgressStyle notifs via test app
Test: patch ag/30006048 and run ProgressStyleNotificationScreenshotTest
Change-Id: Ifc09326cccbe4e84ae49cb620ddb8414bb91aa5a