Add TestAPI for VIService to create AOHD for test

Add test API which configures subsequent AlwaysOnHotwordDetector
creation methods in VoiceInteractionService to use the underlying
test ST module.

Fixes: 276478654
Test: atest CtsVoiceInteractionTestCases
Change-Id: I234199d5c874ff3145ef468cd40be3a07999f09e