VCP: silence gmock warning in test

The huge amount of GMOCK WARNING is making very difficult to look at the
test logs without loosing my patience
Since we don't care about its return value and don't want to enforce a
strict mode, it make sense to use a nice mock

Bug: 372328699
Flag: com.android.bluetooth.flags.aics_api
Test: atest bluetooth_vc_test
Change-Id: I0decbb7f6b36b6f4b9f914591d8dd3689e6b32fe