Move segment splitting by process after converting to drawable segments.

This prepares for the implementation of stretching and rescaling the
drawable segments. The segment splitting by process needs to happen
after the stretching and rescaling, because process should be able to
split the bar anywhere.

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: I4778dfee4addccd3347c391e2c811fa84cd029be