Enforce that NetworkAgentInfo fields are never null.

These fields have been recently audited, confirmed never to be
null, and annotated @NonNull. Ensure that they can never become
null by throwing exceptions in the codepaths that set them.
Also remove some null checks.

Test: atest FrameworksNetTests
Change-Id: I6ce5bb4d69a990f1c857c599b7e50e372352eb87