The Android Emulator is part of Google´s Android SDK and allows you to run a virtualized Android system within a window on your desktop. With the normal behavior write access to the system partition of that Android is prohibited. That can be changed using the adb tool:
$ ~/android-sdk-linux/platform-tools/adb remount
While this temporarily permits changes to /system until the next reboot, any changes will be gone when you close the emulator. If you want the Android Emulator to remember whatever you did to /system, place a copy of system.img in ~/.android/avd/avdname.avd/ and call it system-qemu.img:
$ cp ~/android-sdk-linux/system-images/android-21/google_apis/x86_64/system.img ~/.android/avd/avdname.avd/system-qemu.img
Note that the paths in this are examplary and depend on which Android version you wish to use and how you name you virtual device.