ANDROID: sdcardfs: Add GID Derivation to sdcardfs

This changes sdcardfs to modify the user and group in the
underlying filesystem depending on its usage. Ownership is
set by Android user, and package, as well as if the file is
under obb or cache. Other files can be labeled by extension.
Those values are set via the configfs interace.

To add an entry,
mkdir -p [configfs root]/sdcardfs/extensions/[gid]/[ext]

Bug: 34262585
Change-Id: I4e030ce84f094a678376349b1a96923e5076a0f4
Signed-off-by: Daniel Rosenberg <drosen@google.com>