public boolean isUsable() {
            return isUsable;
        }
Observes Bluetooth adapter state. This responds to user interactions as well as system controlled state changes.
<p>
NOTE: Make sure that this Observable is unsubscribed according to the Activity lifecycle. It internally uses BroadcastReceiver, so
it is required that it us unregistered before onDestroy.
rxandroidble/src/main/java/com/polidea/rxandroidble2/RxBleAdapterStateObservable.java