Add WCT#setAdjacentTaskFragments

Similar to WCT#setAdjacentRoots, but can be called with fragmentTokens
in case the TaskFragments may not be created yet.

Bug: 190433129
Change-Id: I41dd515fb9722ea6e95d0c3fb040cc0d196ad78d
Test: atest WmTests:TaskFragmentOrganizerControllerTest