Exclude from Recents Assistant in half-screen mode

The expectation is that full-screen assistant will not have
FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS attribute, and will be shown in
Recents, whether it's a first or a non-first task. The half-screen
assistant will have this flag set, and will never be shown in Recents.

Bug: 70789568
Test: Check that Assistant with FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS isn't
shown in Recents.

Change-Id: I5bd2f5f638cc4e0100e435d94eee22e7c022aab6