Change android.permission.SERIAL_PORT to signature|system.

    <permission android:name="android.permission.SERIAL_PORT"

        android:label="@string/permlab_serialPort"

        android:description="@string/permdesc_serialPort"

        android:protectionLevel="normal" />

        android:protectionLevel="signature|system" />

    <!-- Allows the holder to access content providers from outside an ApplicationThread.

         This permission is enforced by the ActivityManagerService on the corresponding APIs,