Remove APK observers.

Most app installations have migrated to the new cluster-style layout
where each app is placed in a unique directory.  There are now two
general issues with these observers:

We can't know when a push/sync has finished delivering all splits
for an app.  Also, inotify isn't recursive, so we'd have to manage
watches for each app directory.

Change-Id: I3770fa9059ddf80f933a3804714f2e52591fce49