I moved my apk file to /system/priv-app just as you adviced me in this thread. When I'm trying to install it through clicking this .apk, Android said me that "The application has not been installed". I tried to use adb install myapp.apk for this purpose but again ADB gave me an error: Failure [INSTALL_FAILED_POLICY_REJECTED_PERMISSION] and I suppose that app requires permission which can be granted only by root. So the only way I see now is to run adb shell later su next cd /system/priv-app and install [-cdDsp] [-o USER] [-g GRP] [-m MODE] [-t DIR] [SOURCE]... DEST and that's a point because I totally don't understand this legend:
Copy files and set attributes
-c
Just copy (default)
-d
Create directories
-D
Create leading target directories
-s
Strip symbol table
-p
Preserve date
-o USER Set ownership
-g GRP Set group ownership
-m MODE Set permissions
-t DIR Install to DIR`
How to set attributes to install this apk as system app?