Implement LE L2CAP Fixed Channel

* Implemented LE fixed channel implementation with reference from classic
  fixed channel implementation
* Adjusted LE specific logic in link manager and other related
  components
* Added address type across the LE fixed channel stack
* Adjusted unit test for LE fixed channel cases

Bug: 140938432
Test: bluetooth_test_gd
Change-Id: I7c5cd27e917efa1e72dce05828c26146c10797c2