Shell commands to tweak uid rules on FIREWALL_CHAIN_BACKGROUND

These can be used by manual or automated tests.

Test: Manual by using the `cmd connectivity` shell commands
Test: atest CtsNetTestCases:ConnectivityManagerTest
Test: atest netd_integration_test

Bug: 322562125
Change-Id: I1de55e430141f566e510b37554a9416c4444213b