L2CAP Reassembler: Keep a reference to DynamicChannelImpl

Since we store configuration in ChannelImpl now, we can store a
reference in Reassembler to read the configuration, and in the future,
signal ReqSeq.

Bug: 141557006
Test: bluetooth_test_gd and run_cert.sh
Change-Id: I353f27c3296f289858ea03da7f8f6ddd1af67ad0