Remove the usage of NETID_UNSET from Vpn.java

Modify Vpn#getNetId() to Vpn#getNetwork() and uses NETID_UNSET
when getNetwork() returns null in ConnectivityServiceTest.

Bug: 182963397
Test: atest FrameworksNetTests
Change-Id: I69d449705b1dc541287c72af8dc7705dc4733109
2 files changed