Rename 'Files and media' to 'Files & documents'

Rename the 'Files and media' permission group to 'Files & documents' and
also change the permission group's description.

The motivation for this change is to support the "Media Permission
Update for Android T" project.

Bug: 201318587
Test: manual
Change-Id: Idd7dc35862b6076834e8dedf52be24dd831ce250