Make data warning notification use IEC units

This makes the number presented to the user consistent between
Settings and the notification. Previously the notification used
SI units while Settings used IEC units.

I haven't tested the TYPE_LIMIT_SNOOZED notification as I'm not
sure how to trigger it (though I don't see any reason why it
wouldn't get the same treatment).

Bug: 91635986
Test: manually:
      1) set a limit for 10 MB
      2) use >10 MB of data
      3) verify notification and Settings agree on data use
Change-Id: I31f157332ac273ae260651eb426ab97258180176