Only set embedding rule for top level injection

Only the injected items on the top level pages should always be opened
on the right pane, so we should only register the split rule for that
part. This will leave flexibilities for those injected items in the
subpages.

Fixes: 216026158
Test: Go to Passwords & accounts > {account} > Google Account and see
the page opened in a full screen/new task.

Change-Id: I9f311547854ace410e93fdb4ca394df1079f723c