empathy-protocol-chooser: don't cache TpConnectionManagerProtocol in the store

empathy-protocol-chooser: don't cache TpConnectionManagerProtocol in the store

TpConnectionManagerProtocol are not garanteed to stay valid so we
shouldn't cache them in the store. Instead, we cache the protocol name
and call tp_connection_manager_get_protocol each time we need it.
This should fix crashers as the ones reported in #599386.