chan_pjsip: Provide a means for tracking device state when holding/unholding

Previously PJSIP did not track hold/unhold and it would always simply be
'inuse'. This patch fixes that.

review: https://reviewboard.asterisk.org/r/3129/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@405908 f38db490-d61c-443f-a65b-d21fe96a405b