Fix finicky Notification PendingIntent behavior

This should fix all of the weird problems with the Notification not
being able to launch the OfflineGuidesActivity. I didn't find anywhere
that this was necessary but it definitely doesn't work without it on the
few devices I tested.
Minor sync Notification tweaks

Always display the notification when syncing but remove it if there
isn't a new guide being synced. This is a good compromise between
displaying useful information to the user while not being too needy and
intrusive.