mc-tool: use tp_account_manager_dup_valid_accounts
mc-tool: use tp_account_manager_dup_valid_accounts

tp_account_manager_get_valid_accounts has the confusing
(transfer container) transfer-mode, and is deprecated in telepathy-glib
0.19.9.

Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=55391
Reviewed-by: Xavier Claessens <6f61b6f8ca6f7cbeae6bb087ddf85cf85dce43eb@collabora.co.uk>

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