MocnPostprocessor: Prefer registeredPLMN over cellidentity PLMN

[handymenny]

At least one device, Sony 5 III XQ-BQ52, reports registrationinfo.cellidentity with the same (incorrect) PLMN as ALL_CELL_INFO. I think it's the same for other devices as well, but unfortunately getting feedback for a bug like this isn't easy.

if it turns out that many devices behave this way, we might also consider relying exclusively on registeredPLMN, perhaps using networkoperator as a fallback (Increasing false positives though).

I also added dedicated tests, to test this new Android R+ path and the "dual SIM, same cell and different PLMN" scenario