fm(fix): close filemanager before showReader

This removes filemanager from UI widget stack so it does not listen
to events sent to the reader. Before this patch, screenshot event will
be sent to filemanager.

frontend/apps/filemanager/filemanager.lua

@@ -89,6 +89,7 @@ function FileManager:init()
     self.file_chooser = file_chooser
 
     function file_chooser:onFileSelect(file)  -- luacheck: ignore
+        FileManager.instance:onClose()
         local ReaderUI = require("apps/reader/readerui")
         ReaderUI:showReader(file)
         return true