Implement NotificationProgressBar.processAndConvertToDrawableParts.

Also made a few updates to NotificationProgressDrawable.
  1. Added NotificationProgressDrawable.Point.setColor, which was missed
     in ag/29469473.
  2. For consistency, also updated the drawable to support faded default
     colors.
  3. Added back `equals` and `hashcode` in the Segment and Point class,
     because they are useful for equality assertions in the tests.

Flag: android.app.api_rich_ongoing
Bug: 367805202
Bug: 367804171
Test: atest NotificationProgressBarTest
Change-Id: Ie2b567fa2d2d9aac66bdaf6ac7cbb9c855cd7a2a