Add setters for solid/faded progress segment heights.

Also updated `NotificationProgressBar.getNotificationProgressDrawable`
to public, so that clients of the class can get the drawable and call
these setters.

Flag: android.app.api_rich_ongoing
Bug: 422269932
Test: N/A. Just adding utility functions. Compiles.
Change-Id: I9cd889145fde35f82f21869526c441c102d59b88