Add new llndk library, libneuralnetworks_shim.so

libneuralnetworks_shim.so allows vendor service to run
a NNAPI Driver sAIDL service backed by a NNAPI SL Driver.
SL Driver is a shared library with a known stable C API,
that contains vendor driver implementation.

libneuralnetworks_shim.so is part of com.android.neuralnetworks
APEX package.

Test: run VTS driver sample backed by shim on CF
Bug: 172925288
Change-Id: Ibbb0af00377a2ade212740b61158274a6942be9d