[taskmanager applet] Fix MPRIS entries in context menu for CanPause==false

Test Plan:
Media player entries Start/Pause show matching state and trigger
expected actions both for VLC & MPRIS-patched Gwenview (CanPause=false).

Reviewers: broulik, hein

Reviewed By: hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11007