Rewrite self-handle handling
Rewrite self-handle handling

Use TpChannel's implementation of the self-handle.
If the channel is ready and the group interface is supported, the self handle
is already there.

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control