ReaderActivityIndicator: fix isStub()

it was being overridden by the EventListener:new

caused by #7002

c.f. https://www.mobileread.com/forums/showthread.php?t=335886

frontend/apps/reader/modules/readeractivityindicator.lua

@@ -31,10 +31,10 @@ local EventListener = require("ui/widget/eventlistener")
 local util = require("ffi/util")
 -- lipc
 
-function ReaderActivityIndicator:isStub() return false end
-
 ReaderActivityIndicator = EventListener:new{}
 
+function ReaderActivityIndicator:isStub() return false end
+
 function ReaderActivityIndicator:init()
     if (pcall(require, "liblipclua")) then
         self.lipc_handle = lipc.init("com.github.koreader.activityindicator")