Fix the tablet mode manager

Summary:
Previously we connected the hasTabletModeSwitchChanged to the
tabletModeAvailableChanged which meant that in startup once libinput
signaled us that we have switch, we would toggle that we now have a
tablet mode.

But, we were connecting this to wrong signal, as a result tablet mode
would become available, but actual tablet switch inputspy was never
getting registered, so tablet mode events were silently ignored.

Test Plan:
tested on Dell Inspiron 7000 laptop where tablet mode switch
is available

Reviewers: zzag

Reviewed By: zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27975