Add function to encode PLMNs

Add a function to 3-byte encode PLMN IDs
as specified in 24.008 10.5.1.3, which is
the format used for PLMNs in SIM records.

Bug: 73357413
Test: PlmnActRecordTest
Change-Id: I06c9fed003063746f5f2723efcecfec8c5711211
1 file changed