Add HIDL lifetime and protecting callback info to NN README

This CL copies information from
packages/modules/NeuralNetworks/runtime/VersionedInterfaces.cpp and
modifies the description to be more appropriate for the NN HAL utility
code.

Specific sections added to the README:
* "HIDL Interface Lifetimes across Processes"
* "Protecting Asynchronous Calls across HIDL"

Bug: 170289677
Test: mma
Change-Id: Id381895535d708b627f4746687b4d12e16560639