Revert "Don't use framework strings for formatting file sizes"

The previous revert ag/3403198/ is incomplete.

I just learned that "git revert" has memory.
"git revert" re-applied my previous incorrect merge conflict resolution
when I reverted the same change again.

Test: adb shell am instrument -w -e class
com.android.settings.utils.FileSizeFormatterTest
com.android.settings.tests.unit/android.support.test.runner.AndroidJUnitRunner
Test: make -j RunSettingsRoboTests
Test: manual. GB is shown in Settings > Network > data usage > tap at top
Bug: 70050030
Bug: 70005649
Bug: 36994779
Change-Id: Ia56f8aaf9b5048c102b4e480da0e84e90ee51360