This is needed for devices using configfs, where init listens for
sys.usb.ffs.ready=1 to config usb_gadget. When recovery starts
sideloading, minadbd (forked from recovery) sets the property to trigger
that action.

avc:  denied  { set } for property=sys.usb.ffs.ready pid=541 uid=0 gid=0
scontext=u:r:recovery:s0 tcontext=u:object_r:ffs_prop:s0
tclass=property_service

Bug: 35803743
Test: Device shows up in sideload mode.
Change-Id: Ie7f1224d3a8650160ac29811f73b8286fbced4f4