Limited run-time configuration for media stream keep-alive (thanks Johan Lantz for the suggestion)

[bennylp]

Ticket #883 added compile time configuration for selecting keep-alive mechanism of the media streams. Some applications want more flexibility by configuring this at run-time, ideally by making this part of account settings.

This ticket will add run-time configuration for activating/deactivating stream keep-alive only (no keep-alive packet type and interval settings).

Activating/deactivating stream keep-alive mechanism in run-time

• Make sure that stream keep-alive mechanism is enabled:
  • define PJMEDIA_STREAM_ENABLE_KA to PJMEDIA_STREAM_KA_EMPTY_RTP or PJMEDIA_STREAM_KA_USER in config_site.h
  • optionally, update default keep-alive settings (user packet content and interval) via compile-time macros PJMEDIA_STREAM_KA_USER_PKT and PJMEDIA_STREAM_KA_INTERVAL.
• If you are using PJSUA, configure this via account config:
  • set pjsua_acc_config.use_stream_ka
  • use the account config for adding (pjsua_acc_add()) or modifying (pjsua_acc_modify()) account, so all media streams for that account will use this setting.
• If you are using PJMEDIA level, set stream info pjmedia_stream_info.use_ka and use the stream info for creating stream (pjmedia_stream_create()).

[nanang]

(In [3313]) Closed #1129:

• Added run-time configuration for activating/deactivating stream keep-alive (non-codec-VAD mechanism), also added this config to account settings.
• Fixed bug wrong session info pointer "si" in pjsua_media_channel_update() when call audio index is not zero.