Encode the Number of MNC Digits in CellIdentity

Encode the Number of MNC Digits in CellIdentity

Legacy RIL uses an integer to encode the number of
MNC digits. Because the size is not fixed, leading
zeroes result in ambiguity in the length of the mnc.

This change adds support for passing the number of
encoded digits in the most-significant nibble of the
mnc integer (which is only 10 bits). Thus, on any
implementation that is 16-bits or wider, the mnc info
will be properly encoded and decoded with the
correctly-sized string.

Bug: 111971808
Test: ril::util::mnc::test
Change-Id: I24aeba5328a63f80b0d6b25b068bd19160191dff