AuthGraph: move code into library

Use the core library's new service implementation, which wraps a
channel to the TA.

In this nonsecure case, the TA is local in-process, so use the core
library's AuthGraphTa, and implement the SerializedChannel as just
a direct invocation of the TA.

Move this code into a _nonsecure library, so the main.rs just has
the code needed to start the executable and register the service.

Test: VtsAidlAuthGraphSessionTest
Bug: 284470121
Change-Id: I738d3876872a8cd248f0ebec708676d1173b6e37