Return the RPLMN from NetworkRegistrationInfo

Add an API to get the RPLMN in network registration info.

This PLMN-ID is the one chosen from the PLMN IDs broadcast
by the cell for registration purposes.

Bug: 135921133
Test: make update-api && make offline-sdk-docs && make
      && atest FrameworksTelephonyTests
Change-Id: I82f9150e185d9809572d246b57ea42f14ad64f15