Add annotation for Vpn#getNetwork()

- Add @VisibleForTesting & @Nullable for Vpn#getNetwork().
- Remove null check in caller side(test) of Vpn#getNetwork()
because if the code is working properly, it can never be null.

Bug: 182963397
Test: atest FrameworksNetTests
Change-Id: Ic52864003fbebd9f4e95d43fefc2e168437b0122