Fix kotlin nullable errors in net-test-utils and NetworkStaticLibTests

Fix kotlin nullable errors that were exposed by setting the retention
of android.annotation.NonNull and android.annotation.Nullable to
class retention.

Bug: 294110802
Test: builds
Change-Id: I2a2c91188b64619fca190a9bd7812d32f9610abd
4 files changed
tree: 30a5e1b7d779983196ac43c7843f8871be0a3862
  1. staticlibs/