Multi-window assist callback

Introducing a new callback in VoiceInteractionSession to
provide assist data for additional activities in the
foreground in a multiwindow setup.

PIP, docked windows and free-form windows (top-most)
will be queried for assist data and passed through the
new API to the Voice Interaction service.

Bug: 27718385

Change-Id: Ib4427c304611b75c2078dcb54f1f7e47ae7d9cfa