Methods to calculate user and external disk usage.

Add method to calculate user disk usage, which will be faster than
making a Binder call for every single appId under a user.  Add method
to calculate external disk usage, which uses file extensions to track
usage with "audio", "video", and "images" categories.

Start measuring internal and external storage space separately; new
GIDs are coming in a future CL.  Pass down all package names, inodes,
and code paths, since shared UIDs host more than one.

Test: builds, boots, stats are consistent
Bug: 27948817, 32206268
Change-Id: Icbb82a30fe711ef05e9a16bb1dc740dc943a9ddc