Storage stats on external SD card in Settings.

Storage categories are dynamically created from list of StorageVolumes instead
of a static XML.

Unknown files' sizes are part of Misc rather than Apps.

Categories with a size of 0 are removed.

TODO : remove the notion of a "nosdcard" product. Change strings accordingly.
See all TODO in code

Change-Id: I017ac20f5fa50ad9bdeba8e666754ec84acf3858