[RONs] Implement the structure required for redacting the AOD RON

* This introduces the PromotedNotificationContentModels class which holds both a public and private model
* Implements 2 of 3 redacted content extraction codepaths:
  * Implements unredacted extraction when redaction type is none
  * Implements fully redacted extraction otherwise
  * Does not implement redacted extraction from the Notification.publicVersion object when one is present.
* Does not implement locked state checking for AOD; always shows the public version on AOD for now.

Bug: 400991304
Flag: android.app.ui_rich_ongoing
Test: presubmit
Change-Id: I76086cb35dfdbd396d5b1c226545500f454e40bf