Expose ScanResult#untrusted as a @SystemApi.

- Expose ScanResult#untrusted to inform NetworkRecommendationProviders
  that a ScanResult does not correspond to a saved network.
- Add static construction methods and assertions to RecommendationResult

Test: runtest frameworks-services

Bug: 33490132
Change-Id: If7006040f63843c1c468c9d95c5c017383c5c5dd
Merged-In: If7006040f63843c1c468c9d95c5c017383c5c5dd