Prevent an app to check if the specified VPN app is set VPN always-on

An app can check if the specified VPN app is set VPN always-on
or not by calling VpnManagerService#prepareVpn(). If it sets
oldPackage to null and newPackage to a specified VPN app which
is set always-on, then prepareVpn() will return true. So the app
could know the specified VPN app is installed and it's set
always-on.

Bug: 191276656
Test: atest FrameworksNetTests CtsNetTestCases \
      CtsHostsideNetworkTests:HostsideVpnTests
Change-Id: I0013039f58a6f7c5c73b9acc877d70a0126d0ca7