Don't populate accessibility menu when split selection is active

User should only be able to select the second app to split with, not
dismiss or take any other action.

Fix: 320221290
Test: Enable Switch Access. Open some apps.
Go to Overview and select "Split".
Select the second app. Accessibility menu should not be shown.
Flag: EXEMPT bugfix

Change-Id: I9075e018261949366dec21f64249308601690dbe