Set the restore geometry after placing a ShellClient for the first time

Summary:
The restore geometry gets initially set to 0/0xsize before the placement
is done. When going into updateClientArea and then afterwards into
AbstractClient::checkWorkspacePosition the geometry restory is used for
calculating the new position. This results in windows getting moved to
0/0 when e.g. plugging in a new screen or a panel changes, etc.

This change ensures that the restore geometry is set correctly after the
first placement.

BUG: 366696

Reviewers: KWin, Plasma on Wayland, bshah

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2627