[MS59.1] Add NetworkStatsDataMigrationUtils

Once the data usage part moves into the module, the OEM would
lose the ability to modify the code, thus we need a mechanism
to prevent data loss during migration. Hence, provide a
helper class to read old version of persistent network
statistics, the implementation is intended to be modified
by OEM partners to accommodate their custom changes.

This interface will be invoked by NetworkStatsService during
the boot up phase in the migration process which will be
introduced in follow-up changes.

Test: atest NetworkStatsDataMigrationUtilsTest
Bug: 204830222
Change-Id: I4a9c8a04761b530ce0b31869f0d7fc191b00438f