L2C_CSM: Send ConfigCfm to user after ConfigInd

This way, users know that a ConfigCfm event with status OK means channel
is established. No need to further keep track of state.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I90295349c53d57aa859c5bb65e6f28a299714b3d