Allow Nullable Registered PLMN

API Council has noted an inconsistency
between setRegisteredPlmn() and getRegisteredPlmn().

Because other methods such as CellIdentity#getMncStr()
are already nullable, it's reasonable to allow the
registered PLMN to be null in the event that the device
isn't registered.

Bug: 149476549
Test: make update-api && make && atest NetworkRegistrationInfoTest
Change-Id: Ib38e72a6bdbaf4e1fd439e92f835daa6f87ceb73