I'm wondering is it possible to install .apk file as system app directly (with root)? Without converting user app to system app.
You can simply copy the apk (its name not relevant) into /system/app or /system/priv-app, set the appropriate permissions (rw-r-r, root:root) and then do a reboot. That's it.
Works well on Android 6.0.1.
Via ADB: (Copy apk to phone via usb/My computer first)
adb shell (Get into shell to run beow commands) su (Run as root) mount -o rw,remount /system (Mount system read/write since default it read-only) chmod 755 /system/priv-app/Messaging_SEP81 (Set permissions on folder) chmod -R 644 /system/priv-app/Messaging_SEP81 (Set permissions on apk file) ls -l /system/priv-app/Messaging_SEP81 (Check permissions - Should be: -rw-r--r-- 1 root root - not sure what 1 & 2 is) ls -l /system/priv-app | grep "Messaging" (Check permissions - Should be: drwxr-xr-x 2 root root) reboot (Reboot device)