Add carrier privilege utils to CarrierConfigRule

Move the existing helper methods to acquire carrier privileges in
NetworkAgentTest.kt to CarrierConfigRule where it can be reused by other
CTS tests. Also reset the carrier service package override in the rule
cleanup.

Bug: 216524590
Test: atest NetworkAgentTest
Change-Id: Id907a2d641aa61838119e2be65654b9796464498
2 files changed