[Applet]Improvements for unavailable port.

Summary:
Restore capability of unavailable ports to be active, which was prevented
with 10ab204a.
Enable port selection in case an unavailable port is active.

Test Plan:
Before: Choosing an unavailable port from pavucontrol was impossible.
After: Activating an unavailable port is possible. Can test with D27422.

Reviewers: #plasma, drosca, ngraham

Reviewed By: ngraham

Subscribers: pino, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27420