Reset adjacent TaskFragments while updating split container

The TaskFragments were set to adjacent only when registering
a new split container.

It should also be done while updating the split container, like
when the user navigated back and a TaskFragment was removed.

Bug: 200504061
Test: repo steps on the bug
Change-Id: I432662fd098c98d96f8b2fd5fc4277f77be430d7