Call problems between SRTP optional-with-duplicated-offer and SRTP disabled (thanks Bogdan Krakowski for the report)

[nanang]

Problems reported:

1. no audio after call established
2. crash when trying to re-invite/hold/update.

Steps to reproduce using pjsua app:

1. instantiate pjsua A with SRTP mode 3 (optional with duplicated m lines in the offer)
2. instantiate another pjsua B with SRTP mode 0 (disabled)
3. pjsua A calls pjsua B, the first problem occurs here, pjsua A choose the encrypted media to be the active media (before call established), which later it is deactivated by pjsua B
4. pjsua B sends re-invite/hold/update, the second problem occurs here, when SRTP transport_encode_sdp() accessing second media lines while actually only one media line generated.

[nanang]

(In [3334]) Fix #1143:

• Fixed no audio bug when pjsua with SRTP optional-with-duplicated-offer calls pjsua with SRTP disabled, by updating active media index after SDP negotiation done.
• Fixed bug in generating SDP, pjsua_media_channel_create_sdp(), by making sure all media in the SDP candidate are aligned with current active SDP before calling pjmedia_transport_encode_sdp().
• Fixed bug in modifying SDP for call hold, the media index to be modified was hardcoded to 0, should be active media index.
• Added python tests for calls with SRTP optional-with-duplicated-offer.