[Applet] Fix empty space and give correct size

Summary:
Make default size same with system tray. Inside area is bigger,
without the system tray heading.

Fix empty space.

Test Plan:
Use applet outside the system tray on taskbar or desktop with
display scaling.

Bug: 393418
Bug: 413448

Reviewers: #vdg, #plasma, drosca, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26548