ofonod: keep VoiceCallManager published across imsd restarts; enable on install

GNOME Calls 50.0 builds one origin per appearance of the modem's
VoiceCallManager interface and never disconnects a removed origin's
CallAdded handler, so every withdraw/re-add cycle leaves a stale handler
behind: calls announced twice after one cycle, a SIGBUS on freed memory
after a few (measured on a Fairphone 6). Withdrawing the interface on
every imsd restart or registration drop therefore cost the only incoming
call path on Phosh, where Calls runs as the session's daemon.

List the interface permanently. Registration is reported through the
modem's Online property and enforced in Dial, which now fails with
"imsd is not registered" or "imsd is not running". Only this daemon's own
restart (org.ofono vanishing) still cycles the origin, and the README says
to restart Calls after that.

Installing the opt-in imsd-ofono subpackage now enables imsd-ofonod (a
systemd preset plus the wants symlink, like imsd.service in the main
package), and imsd-dialerd's autostart is limited to KDE so a Phosh
session does not run the Plasma shim.

Verified on the FP6 under Phosh with GNOME Calls' autostarted daemon:
outgoing call, incoming call answered on the lock screen, then an imsd
restart under the running Calls (Online changes only, no origin cycle)
followed by another incoming call added once and answered.

packaging/imsd-dialerd.desktop

@@ -7,5 +7,6 @@ Comment=Plasma Dialer backend for imsd (replaces plasma-dialer's modem-daemon)
 Exec=/usr/bin/imsd-dialerd
 Icon=call-start
 NoDisplay=true
+OnlyShowIn=KDE;
 X-KDE-autostart-phase=1
 X-KDE-StartupNotify=false