voip_calls: Get rid of global variables.

Move the members of voip_rtp_tapinfo_t to voip_calls_tapinfo_t. Move a
bunch of formerly static variables there as well. Note that we might
want to make them private again at some point. Remove launch_count. It
appeared to be unused.

Wrap the individual registration routines in voip_calls_init_all_taps.
Wrap the removal routines in voip_calls_remove_all_tap_listeners.

Move voip_calls_get_info() to voip_calls_dlg.c. Make it static.

Change-Id: I58f9d91f55cfb5e4b0c048a5a1d5d4b947f4641b
Reviewed-on: https://code.wireshark.org/review/5372
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>