libnetd_test_unsol_service - switch to netd_aidl_interface-unstable-cpp

This is needed to resolve aidl version mismatch between
netd_integration_test and libnetd_test_unsol_service.

Test: builds
Bug: 157869568
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I756a50454c60e7329dd43eff8a4235a27b864ee3