rework ringer to allow silencing

The main motivation for this refactor was to easily allow silencing a ringing call.

Furthermore it should fix #339 (closed), although that is hard to test as I've never encountered the issue myself. Also I would've expected this issue to also show up with phosh as a DBus consumer (and not necessarily only when hanging up, possibly also when answering a call).

So in short I hope reworking the ringer helps alleviate issues related to ringing:

• #339 (closed) (by making sure ringing stops when it's supposed to)
• #93 (closed) (ringing starts when it's supposed to)
• #195 (closed) (this was actually fixed by https://source.puri.sm/Librem5/feedbackd/-/merge_requests/67)
• #169 (closed) (by not triggering an event more than once)

Furthermore the ringer should be covered with some testing (increasing code coverage by a whooping 3% :p ).