Dismiss dream when activities are started while dreaming.

This introduces a new config which allows per-device configuration of
this behavior by activity type. By default, we allow assistant
activities over dreams.

Fix: 198041170
Fix: 230112107
Fix: 228421066
Fix: 229120775
Test: locally on device
Change-Id: I2851f11da19e7c2ac346e2ecb80d66321fc3765e