Make firewall bpf code multi-user aware

The check is_system_uid should mirror the definition of core_uids in
UserHandle.isCore until it is available to be read from mainline code.
isUidNetworkingBlocked should also do the same.

Test: All existing tests should pass.
Test: atest CtsHostsideNetworkPolicyTests
Test: atest FrameworksNetTests

Bug: 348316140
Change-Id: Id5e01289b301a725b162ef249062e0f2806876dc
3 files changed