L2CAP: Simplify parameters for segmenter and reassembler

Currently remote_cid and queue_down_end can be fetched from ChannelImpl.
Remove unnecessary parameters for better cohesion.

To make it work, we cannot mock AllocateFixedChannel() in Link.
Since Link needs to allocate a fixed channel for signalling manager
during ctor, it doesn't make sense to make AllocateFixedChannel()
virtual.
Since the coupling between Link and Signalling Manager is too tight, we
are unable to test signalling manager by itself. Instead use cert test.

Bug: 144503952
Bug: 141557006
Test: run_cert.sh and bluetooth_test_gd
Change-Id: I4be323bc7a9810f5f6b500b4f0f0fc4fcad12b6c