Add (nonsecure) default AuthGraph impl

Add an implementation for the AuthGraph key exchange HAL, to allow
testing and policy compliance.

In real use, IAuthGraphKeyExchange instances are expected to be
retrieved from some other HAL, allowing the AuthGraph instance to be
specifically correlated with the component that uses it.

Bug: 284470121
Bug: 291228560
Test: hal_implementation_test
Test: VtsAidlAuthGraphSessionTest
Change-Id: Ib064292d60bead663af7721fd1406f2a9b5d8ecd