L2CAP Segmenter: Keep a reference to DynamicChannelImpl

Since we store configuration in ChannelImpl now, we can store a
reference in Segmenter to read the configuration, and in the future,
observe I-frame and S-frame changes from remote via ChannelImpl.

Bug: 141557006
Test: bluetooth_test_gd and run_cert.sh
Change-Id: Iba0c6ee1dd915e7ef7951f4208abb8467cb9755d