Show sensor usage dialog for assistant

The problem is that the assistant doesn't draw its own
UI in some cases so even though it's in the background the user
considers it as a foreground app. We can check if the app trying to
access the microphone currently has a voice interaction session which
should be a good indicator that the user intends to user the app.

Test: Use the assistant, observe dialog
Bug: 178243539
Change-Id: Ie17f6e47a002e75467cce23fd74f5fcbc92aa577