Stop using mVpns in getConnectionOwnerUid.

Use data that is already available in ConnectivityService
instead.

The behaviour of the new implementation is slightly different
from Q and R code when the permission check fails.

- The old code would throw a SecurityException if an app that
  was not an active VPN called the method, and would return
  INVALID_UID if the connection belonged to a UID that was not
  subject to the VPN.
- The new code returns INVALID_UID in both cases.

This does not seem like a compatibility problem. The only case in
which the code throws SecurityException is if the app is not a
current VPN app, but the app already knows whether it is or not.
The docs don't mention that the method SecurityException, either.

Bug: 173331190
Test: atest FrameworksNetTests
Test: atest HostsideVpnTests
Change-Id: If3d031e74df33b5c97e12ebf02272faac6769d50