Before android 4.4 kitkat, we can just put the app in /system/app, the app can has the system permission. However, from 4.4, the new location is changed to /system/priv-app. It costs me several days to figure out why my old app can not work.
For a clean nexus 4, I flash the recovery mode and restart the phone. It stuck at the begin X, even after I waited 10mins.
–wipe the cache and all data, when first go to the recovery mode, it shows some error for cache part. After wipe twice, the system back to life.
Another way to find the pkgusagestats is /data/system/usagestats/usage-*