Include GIDs for unenforced permissions.

When READ_EXTERNAL_STORAGE isn't enforced, grant its GID to all
launched processes.  When changing enforcement, kill all processes
below foreground adjustment, causing them to be relaunched with
update GIDs.

Bug: 6131916
Change-Id: I6d83efc937919f13a1a7d9caac902e572869406a