Only call onThreadEnableStateChanged() when the callback is registered
or the state has changed.
Bug: 342999416
Test: atest CtsThreadNetworkTestCases:android.net.thread.cts.ThreadNetworkControllerTest#subscribeThreadEnableState_getActiveDataset_onThreadEnableStateChangedNotCalled
Change-Id: I822d2b98b221c3312ff472b82ebb3e7e88e313fe