Fix AssistantStackTests for when assistant is AOT

When the assistant is configured to be always on top, it will not
allow any other activity to start aboveor under it (unless it is
translucent).
This CL adjusts the tests to reflect this.

Bug: 158117282

Test: atest AssistantStackTests
Change-Id: Iaf51dfa8b489dbd754399adeb4030db6d892efff
2 files changed