McdDispatcher: rewrite logic for whether channels bypass approval
McdDispatcher: rewrite logic for whether channels bypass approval

It doesn't make sense to check whether each individual channel can bypass
approval - what we want is to see whether the whole batch bypasses
approval. Also, instead of calling get_default_handler() we can just
iterate through the saved list of possible handlers.

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