Create a new stack for the assistant activity.

- Add a new stack that is not resized with multiwindow, and
  appears above the fullscreen and docked stacks, but below
  the pinned stack
- Add a method on VoiceInteractionSession to allow the assistant
  to launch activities into this new fullscreen stack.
- Also prevent any activities in the assist stack from the
  fetching of the on screen assist data.

Bug: 30999386
Test: android.server.cts.ActivityManagerAssistantStackTests

Change-Id: I22ab7629b5f758cf1e66d7d1c26648af6bc887c9