L2cap user: Use ConfigCfm_Cb as channel open indicator

When the user receives ConfigCfm_Cb, it's guaranteed that ConfigInd_Cb
is received with correct option enforced.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I6de12f1ba6e9612f6a84b399e7fc984a7f3c3c72