Sending data to bonded devices with Service Changed char does not work on 2.3.0 o Android 6 or lower

[philips77]

The issue is related to https://github.com/NordicSemiconductor/Android-nRF-Toolbox/blob/master/app/src/main/java/no/nordicsemi/android/nrftoolbox/profile/BleManager.java#L391. The ensureServiceChangedEnabled() method is called by ``nextRequest()which sets themOperationInProgress``` flag to true. Adding another request in the line mentioned above is an error, as it will not be processed. Instead ```internalEnableNotifications(...)``` should be called.