Add VIMService model enrollment override

In order to test voiceinteraction, we must be able to override the
persistent model enrollment database with an in-memory equivalent.
- Abstract DatabaseHelper into IEnrolledModelDb interface
- Implement the Test version of this interface as a hashmap
- Add VIMService APIs and client-side equivalent to override the Db with
the test version
- Remove incorrect GuardedBy annotations (no GuardedBy(this) on a
  public interface boundary)

Test: atest
AlwaysOnHotwordDetectorTest#testAlwaysOnHotwordDetector_startRecognitionWithData
Fixes: 273309878

Change-Id: Icf4f1aaf147519ef45534ee8c7ae26c6040fa4cd