Improve nullability of CarrierPrivilegesTracker

- Add @Nullable and @NonNull annotation to both internal states
and method parameters and return type.
- Make sure all @Nullable variables have null pointer check before
  usage to prevent NPE.

Bug: 217442920
Test: CarrierPrivilegesTrackerTest
Change-Id: Ib6a7150750604c9115c00235b7a5e22c56e65012