Fix the bytes displayed unit

This used to by MByte and GByte, not used in other places in Settings,
and not align with the data summary on the same page.

Unify to MB and GB to align with other places in Settings and the data
summary on the same page.

Fix: 277900792
Test: Visual
Test: Unit test
Change-Id: I9f8ceac470aede135b7921c391c063ba87da661e