L2CAP: Make a common ChannelImpl interface

This allows passing the ChannelImpl to reassembler and segmenter,
instead of passing nullptr for fixed channel. This reduces some
duplicated information in parameters.

Bug: 144503952
Bug: 141557006
Test: bluetooth_test_gd and run_cert.sh
Change-Id: I737488ed1f6d6f985bc7dc45a84ce86fb6723b50