Solved progress bar & thread problems when install

Added a flag to let the system know a thread of loading items is running
so it won't be called to run again to mess up the re-installation of this
app.
Stop re-loading when user "backs" after viewing

Added some codes to stop re-loading the picture/pdf files when user hits
"back" key after viewing it, since there's no add/delete function in this
current APP, it seems it's reasonable to stop reloading/refreshing the
files when user backs out from viewing it, so it saves time and imporves
user experience.