Stop using TpChannelIface properties

As mozilla removed certain hooks from its API, gjs had to change the way properties on introspected objects work - as a side effect, it now relies more heavily on information provided by the typelib.

However TelepathyGLib doesn't expose most interfaces to introspection, so their properties are now invisible to us. In theory this is easy to fix - just pass the files in question to g-ir-scanner - but in practice telepathy-glib hasn't seen any activity in years, so getting a patch committed (and released in a tarball) is nigh impossible.

So instead, address this by using the corresponding accessor functions instead of directly using the properties provided by affected interfaces.