Stop MockVpn from extending Vpn class.

Make MockVpn stop extending Vpn in order to stop depending on
Vpn code. This includes:

1. Remove @Override and synchronized for all MockVpn methods.
2. Remove the constructor.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: I9109cd9d3e17717cffa6c89c6a9e4330ed9af3cf
1 file changed