Autogenerate toString for networkstack/ipmemorystore parcelables.

Also add a couple of tests for the output of the toString method
on select parcelables.

Because this is an API change, also freeze the AIDL interfaces in
order to make the tests pass in mainline-prod and master, which
use the latest stable version of the interfaces.

Bug: 131797393
Bug: 150161123
Fix: 173203513
Test: new unit tests
Change-Id: I145a548a94dd9a72bf192fe5058e142b46bc8474