Introduce WCT#clearAdjacentRoots()

This CL adds clearAdjacentRoots() to WCT as the “inversion” of
setAdjacentRoots().
ARC needs this capability because adjacency of the split roots
depends on the device status and it should be possible to switch
that in runtime.

Bug: 245239136
Test: WindowOrganizerTests
Change-Id: I59fb28508d56973fe4643f4c4a7deee6d29cbb4f