Fade away half-screen assistant on swipe-to-recents gesture.

Half-screen assistant is not in task stack, and will be faded away.

Workspace-to-recents case with half-screen assistant needs more work
after we decide whether we should swipe to the workspace preview or to
the first task.

Bug: 70789568
Test: Manual: swipe a normal app to Recents; swipe an app with a
half-screen assistant on top of it to Recents.

Change-Id: I6e3acc2b94bb3b56d6590a72073d37a1771a1da5