installing apk in /system/app with android 5.1 or higher (simple adb push doesn't work anynore)
In Android API Level 21+ (5.1+) modifications in /system/app will not be recognized by the package manager automatically. Insteadyou should try /system/priv-app.
Assume your app is called Foo.apk, create a folder in /system/priv-app/Foo and set chmod 755 &amp; chown 0.0 to it. Then put your apk file in it (the path should be /system/priv-app/Foo/Foo.apk) and chmod 644 &amp; chown 0.0 to it. It will be recognized by the package manager then, or if not just reboot you device and the app is there.
In some ROMs mods in /system/app WILL be recognized automatically but it depends.