Only extract RON content model for RON style notifications.

Created Notification.RichOngoingStyle, which is currently empty. This
class is supposed to be the base class  for the various RON use cases.

The tests were performed with the desktop app modified to use the new
style for running timer notifications.

Flag: android.app.api_rich_ongoing
Flag: com.android.systemui.notification_row_content_binder_refactor
Bug: 337261753
Bug: 343942780
Test: Trigger running timer; observe notification from the content model.
Test: Miss timer; observe timer notification not from the content model.
Change-Id: I10983db618dc212562ff6c333d4f602a76aaf484