Add tests for always on lockdown VPN on system user.

Add test coverage for Vpn.setAlwaysOnPackage() where the caller
is system user to ensure uid = 0 is not restricted.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: Id9f81fdf0147597f64f8440b971930b3bd7b55e5
1 file changed