How to set a system property from an android app?

I need to do setprop but from an app.

Right now I'm trying to just run it as a shell command:

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "test:MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        exec("setprop service.adb.tcp.port 5555");
    }

    private void exec(String cmd) {
        Process proc = null;
        BufferedReader successResult = null;
        BufferedReader errorResult = null;

        Runtime runtime = Runtime.getRuntime();
        Log.d(TAG, "exec command: " + cmd);
        try {
            proc = runtime.exec(cmd);
            successResult = new BufferedReader(new InputStreamReader(proc.getInputStream()));
            errorResult = new BufferedReader(new InputStreamReader(proc.getErrorStream()));
            String s;
            while ((s = successResult.readLine()) != null) {
                Log.d(TAG, "exec command: " + s);
            }
            while ((s = errorResult.readLine()) != null) {
                Log.d(TAG, "exec command: " + s);
            }
        } catch (IOException e) {
            Log.e(TAG, "Error running shell command:", e);
        } finally {
            try {
                if (successResult != null) {
                    successResult.close();
                }
                if (errorResult != null) {
                    errorResult.close();
                }
            } catch (IOException e) {
                Log.e(TAG, "Error running shell command:", e);
            }

            if (proc != null) {
                proc.destroy();
            }
        }
    }
}

This give me:

exec command: setprop service.adb.tcp.port 5555

exec command: setprop: failed to set property 'service.adb.tcp.port' to '5555'

How can I set a system property from an application?

Edit: Sorry, I should have I added my device is rooted and this app is a system app.