Correctly update the z-order list on display move

Previously, the taskId was not removed from
DesktopRepository#freeformTasksInZOrder when a task was moved from one display
to another. This commit addresses that issue.

Bug: 381470076
Test: DesktopRepositoryTest
Flag: EXEMPT bugfix
Change-Id: I2721405781e2d04c0577aa9bf80c7d1dcc88ce71