[NGA v2] Reliable Visible Activity Lookup

To enable more voice-oriented in-app user journeys, Google Assistant
needs to access more information regarding the visible activities
via the Voice Interaction Session. Thus, we provide the APIs to receive
the changed visible activities in Voice Interaction Session.

Bug: 178244510
Test: atest CtsVoiceInteractionTestCases
Test: atest CtsVoiceInteractionTestCases --instant
Change-Id: I019d09fee8105ae1eadbc76803f46fd8b1948f6b