Update file size formatting.

Per UX, default strings should have space between value and units
resulting in "12.3 GB".  Add a formatting variant that returns the
various components for callers who want to build their own strings.

For now there is only one mounted emulated volume at a time, and
it's always the primary storage, so give it the default rootId to
keep old Uris working.

Change-Id: Ifcc72a91a6b397ee65dc92642153286186eb64ac