Add DataUnit to clarify SI-vs-IEC units.

Mirrors the design of TimeUnit and ChronoUnit which many developers
are already familiar with, making it easy to pick up and use.

Yes, this is an enum.

Bug: 70915728
Test: bit FrameworksCoreTests:android.util.DataUnitTest
Change-Id: Id0cfdac5c81ed89c3c9ece23c964acba4a4f8471