Fix net_test_bta_security linking errors

When modifying some unrelated tests, I started getting linking errors on
net_test_bta_security.

Instead of re-defining some methods as empty, use proper mocks.

Bug: 340188796
Test: atest net_test_bta_security
Flag: exempt, trivial
Change-Id: I7f9e759a252df2e23e85662f3514411478724d7c