Add tests for onUserAdded and onUserRemoved

Test coverage of adding and removing a restricted user with and
without lockdown mode, using either setLockdown() and
setAlwaysOnPackage().

This change also refactors makeVpnUidRange() to return a list
and adds makeVpnUidRangeSet().

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: I47a25e9f0337f5c1d5754c279534640cd2753b5c