[SB] Add helper function for MobileConnectionRepository.numberOfLevels

Now that carrier merged connections also need to inflate the number of
levels, we should provide a helper function to make that inflation
uniform across all mobile classes.

This also lets us not re-implement logic in the demo classes.

Bug: 423686800
Flag: EXEMPT refactor
Test: atest MobileConnectionRepositoryImplTest
Change-Id: I4e83bcb06b2ab58bd8d20e7734ddcc7fea4b4ec0