New firewall chain for default background restrictions

A new firewall chain is needed to configure background network
restrictions for apps.
This change only adds the API stubs and traffic controller constants to
make the chain work. Policy changes using this chain will follow in
the framework code.

Test: atest CtsNetTestCases:ConnectivityManagerTest
Test: atest ConnectivityServiceTest

NO_IFTTT=The Lint rule along with the relevant code in Common.h is
being deleted in aosp/2819759

Bug: 304347838
Change-Id: I33e2db6671431f7c576fc931d9f96e684fc1e78a