Add CarBuiltinLibUnitTest to visiblity.

We move some of the unit tests for car-builtin-lib to a different
folder. It needs access to bluetooth hidden API implementation.

Flag: EXEMPT build rule
Test: atest CarBuiltinLibUnitTest
Bug: 346425841
Ignore-AOSP-First: Submit with internal refactor together

Change-Id: Ifaf646faa4a87466140ea7bedc37e067a4e40b17