Fix NPE in AR hardware binding sequence.

b/22772433

To keep the old Watcher binding sequence backwards compatible,
it is required to ensure that the callback is only invoked if
the feature is supported by the platform, and an instance is
available.

Change-Id: I0a6335f10647184abdad3ee1583b505160a3308f

services/core/java/com/android/server/location/ActivityRecognitionProxy.java

@@ -123,6 +123,12 @@ public class ActivityRecognitionProxy {
                 Log.e(TAG, "No watcher found on connection.");
                 return;
             }
+            if (mInstance == null) {
+                // to keep backwards compatibility do not update the watcher when there is no
+                // instance available, or it will cause an NPE
+                Log.d(TAG, "AR HW instance not available, binding will be a no-op.");
+                return;
+            }
             try {
                 watcher.onInstanceChanged(mInstance);
             } catch (RemoteException e) {