The navigation sample app uses com.mapquest:search-ahead-v3:1.2.9@aar which has a resource value search_ahead_uri defined as http://searchahead-public-api-b2c-staging.cloud.mapquest.com/search/v3/p... which does not exist. It should be: http://www.mapquestapi.com/search/v3/prediction according to your developer docs.
What version of the SDK has a fix for this problem? Also, is there a newer version of the sample app that's been updated to use your latest SDK? I'm in the process of evaluating possible SDKs to use for an embedded mapping/navigation app and cannot afford the time to debug sample apps. Can you please provide an answer within the next day or two?