Thread-safe Android BT LE Java implementation

The JNI calls from Qt/C++ to Java are typically executed in different thread than the Java-side callback functions.

This means that there needs to be proper thread protection mechanisms in place on both Client and Server side.

As an example the JNI disconnectServer() call might null the mGattServer variable while a callback using the mGattServer is in progress in another thread. This can lead to unpredictable runtime behaviors.