Although OP seems to have SuperSU installed, I am writing this answer for the users of Magisk so as to prevent near duplicate questions from spawning.
Magisk module App Systemizer can make any third-party app a system app systemlessly i.e. without making changes into system partition. Download it from Magisk Manager or via the link and flash it through Magisk or your custom recovery. Reboot and launch a shell using a terminal app, such as Terminal Emulator. Follow these commands:
systemize # loads the tool
1 # this would list all the third-party apps that can be systemized
ENTER_DIGIT # enter the digit corresponding to the app you want systemized
CHOOSE_LOCATION # choose the digit corresponding to /system/app or /system/priv-app
TYPE y OR n # y to reboot immediately and n to reboot later
After reboot your app would show up as a system app. When you get comfortable with this tool you can direct everything through a single command. Follow the module's link for usage instructions.