Fix L2CAP mocks in rfcomm-fuzzer

With the switch to using an l2cap interface, the previous fake l2cap setup was no longer working in rfcomm-fuzzer. Mock the affected methods using the interface mock rather than the legacy fake stack which is no longer valid.

Bug: 381373367
Change-Id: Ia06802476bb12277e76a7c1a1f40cca1ae904007
Test: m rfcomm-fuzzer
Test: FUZZER_NAME=rfcomm-fuzzer ${ANDROID_HOST_OUT}/fuzz/$(get_build_var HOST_ARCH)/${FUZZER_NAME}/${FUZZER_NAME} runs without segfault
Flag: TEST_ONLY