Rename Workspace::updateClientArea as Workspace::rearrange

We need to re-arrange layer shell surfaces, compute new struts and adjust
the windows in a single step.

Workspace::updateClientArea() is the best candidate for that, so this change
repurposes that function from computing work areas to a generic relayouting
function.

CCBUG: 482361
(cherry picked from commit b674b458)