recovery.te: Allow setting sys.usb.ffs.ready.

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

public/recovery.te

@@ -105,6 +105,9 @@ recovery_only(`
   # Read serial number of the device from system properties
   get_prop(recovery, serialno_prop)
 
+  # Set sys.usb.ffs.ready when starting minadbd for sideload.
+  set_prop(recovery, ffs_prop)
+
   # Use setfscreatecon() to label files for OTA updates.
   allow recovery self:process setfscreate;