How to install a system app in Android 5.0 or later devices ?

Rename your apk file to base.apk

$ adb push base.apk /sdcard/

$ adb shell

$ su

$ mount -o remount,rw -t yaffs2 /dev/block/mtdblock3 /system

$ mkdir /system/app/Test

$ chmod 755 /system/app/Test

$ cat /sdcard/base.apk > /system/app/Test/Base.apk

$chmod 644 /system/app/Test/Base.apk

$ mount -o remount,ro -t yaffs2 /dev/block/mtdblock3 /system

$ exit

Reboot your device. When boot completes you should see a system message like Android updating ...