Define a category for "file management" apps.

We already have a nice collection of CATEGORY_APP_* constants that
can be used to identify classes of apps.  This change expands that
list to discover file management apps, such as DocumentsUI.

Bug: 130215653
Test: none
Exempt-From-Owner-Approval: trivial manifest change
Change-Id: Id2d234615cf78f5373d8d3a389b1b089668eef64