Display total storage for Internal shared storage.

Prior to this change, Settings was using
volume.getPath().getTotalBytes() to calculate the total, which includes
the user-accessible storage, but not the internal partitions. As such,
the total displayed was often missing many GBs (for example, in a 32GB
device, it could display "12.09GB of 25.01GB used".

This change fix this problem by using the total size and creating a
"System" section containing the "missing" storage.

BUG: 24128505

Change-Id: Ic35cd7c8406eff16ac1d97c4b4c233ecde64a6e1

** Cherrypicked from master - DO NOT MERGE **