Add setSubscriptionPlans() and getSubscriptionPlans() @SystemApi

Add two interfaces to communicate with NetworkPolicyManagerService
and make them @SystemApi for mainline support.

Bug: 138306002
Test: atest FrameworksNetTests
      atest FrameworksTelephonyTests

Change-Id: I9f1168bbc70dce9b2b107e1f946737b1d85599c7