chan_pjsip: Use session for retrieving CHANNEL() information.

chan_pjsip: Use session for retrieving CHANNEL() information.

The CHANNEL() dialplan function implementation for PJSIP allows
querying of PJSIP specific information. This used the channel
passed in to get the PJSIP session and associated information.
It is possible for this channel to be masqueraded and end
up as a different channel type by the time the information
request is actually acted upon.

This change retrieves the PJSIP session safely and accesses
data from it (including channel). This provides a guarantee
that the session and channel will not be altered when the
request is being acted upon.

ASTERISK-26673

Change-Id: I335e12b89e1820cafdd92b3e7526b8ba649eb7e6