btm/iso: Add cis connecting state

Track connecting state of the CIS, since LeAudio can call DisconnectCis
when CIS is pending. This can happen, when controller does not send HCI
CIS Established event for a long time.

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
test: atest --host  net_test_btm_iso

Change-Id: I6973b0835a7ccd736a1166b62941cfce6b1df4f1