[client] Properly track enteredSurface in Keyboard
Summary:
ASAN found a heap-use-after-free when deleting the focused keyboard
surface in the client library. Keyboard did not track the lifetime of
the focused surface and thus one can access already freed memory.
Test Plan: Adjusted auto test to verify the variable gets cleared
Reviewers: Frameworks, Plasma
Subscribers: plasma-devel
Tags: Plasma on Wayland, Frameworks
Differential Revision: https://phabricator.kde.org/D6741