Keep the focus on the primary when launch ActivityEmbedding placeholder

Default focus should be on primaryActivity when a placeholder is shown.
This is a short term improvement as focus navigation works from primary
to secondary but not in reverse.

Fix: 229138140
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Change-Id: Ic4e2280e4b37a8fa04fc532245a5d0cfa4e5e012