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/
........

Merged revisions 405908 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405910 65c4cc65-6c06-0410-ace0-fbb531ad65f3