Fix crash in l2cap-fuzzer

l2cap-fuzzer initializes itself via L2CA_RegisterWithSecurity, which calls through into BTM_SetSecurityLevel.  For the client interface defined for these fuzzers, BTM_SetSecurityLevel is nullptr, which leads to a crash on startup.

Add a mock for this in the default client interface.

Bug: 326853665
Test: m libbluetooth
Test: l2cap-fuzzer
Flag: EXEMPT test_only
Change-Id: I282db97b683efb1ba2596d4fad0070215f4cb62c