Consolidate adding add-on directories

This builds off of hrev46243 adding add-on directories all in one place
in AddOnMonitorHandler instead of repeating the code 3 times in
IndexServer, AddOnManager, and MediaAddOnServer.

The safe mode checking in InputServer is now redundant since it all
gets funneled into AddOnMonitorHandler::AddAddOnDirectories()
and the safe mode flags are checked there.

We should probably remove the InputServer::SafeMode() method, but,
I didn't want to break anything that depended on it so I left it.