Improved storage accounting:

- Removed duplicated entry for DIRECTORY_MOVIES.
- Added DIRECTORY_DOWNLOADS to Other.
- Added unaccounted data to Other.
- Changed Other description so it mention non-visible items.

Often sub-directories of /data are populated with data that Settings
doesn't have control of (for example, on /data/ramdump), so they're more
suitable to be displayed in the "Other" section.

BUG: 31091863
Fixes: 30952263

Change-Id: Ibf54f9dd96456575109e2e281f3780da84f70d92