Let the system server have CONNECTIVITY_USE_RESTRICTED_NETWORKS.

Also :
- Fix testUidFilteringDuringVpnConnectDisconnectAndUidUpdates that
  was failing on devices with a first released SDK >= Q
- Add a test actually tests that the system has the permission, as
  the test was only testing what's in the mock

Bug: 119770201
Test: New test making sure this stays true
Change-Id: I74cf5f0fa17fcf818f1fed78c7e3e4375c20152e