interfaces: fix incorrect signature of ofono DBusPermanentSlot (2.29)

[zyga]

This branch addresses recently introduced bug in the ofono interface.
The interface had a buggy specification method signature that would be silently unnoticed
and would result in incorrect confinement at runtime.

[jdstrand]

Thanks for fixing this. I'll let others comment on the test changes but cursory look, +1.

[stolowski]

+1

[zyga]

The history here is wrong. I'll correct that. EDIT: done now.

[codecov-io]

Codecov Report

Merging #4123 into release/2.29 will not change coverage.
The diff coverage is 77.3%.

@@             Coverage Diff              @@
##           release/2.29   #4123   +/-   ##
============================================
  Coverage          75.8%   75.8%           
============================================
  Files               433     433           
  Lines             37250   37250           
============================================
  Hits              28236   28236           
  Misses             7042    7042           
  Partials           1972    1972

Impacted Files	Coverage Δ
overlord/configstate/config/transaction.go	84.45% <ø> (ø)	⬆️
overlord/snapstate/flags.go	100% <ø> (ø)	⬆️
interfaces/ifacetest/backend.go	0% <0%> (ø)	⬆️
interfaces/builtin/ofono.go	73.91% <0%> (ø)	⬆️
cmd/snap/main.go	63.58% <0%> (ø)	⬆️
cmd/snapctl/main.go	36.36% <0%> (ø)	⬆️
spdx/parser.go	84.26% <0%> (ø)	⬆️
interfaces/ifacetest/backendtest.go	0% <0%> (ø)	⬆️
cmd/snap-update-ns/main.go	0% <0%> (ø)	⬆️
overlord/configstate/configmgr.go	0% <0%> (ø)	⬆️
... and 69 more

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