Close assistant activities on closeSystemDialogs

When the assistant is configured to be on top of the dream, it will have
higher z-order than any other activity. If it is also opaque, it will
prevent other activities from starting. We want to close the assistant
on closeSystemDialogs to allow other activities to start, e.g. on home
button press

Bug: 158117282

Test: atest AssistantStackTests
Change-Id: I226d95b82c1f5df5e5327ef85859d9b137d6e0e7