Extending Thread is really not the way to do concurrency these days (and never really was). I prefer AsyncTask for background operations, which has the onPostExecute method from where you can trigger UI updates.
As to your code, have you made sure that the run method is actually executed, and that the value of "PPGresult" is what you expect it to be? Put a log statement first thing in method that prints out its value.
I also note that you seem have two variables named "PPGresult", one in TestJNIActivity and one in SensorInterface. That's bound to cause confusion. It'll also be a source of concurrency issues due to variable visibility rules. At the least, declare variables accessed from different threads to be volatile.
As an aside, it is customary for Java variables to start with a lowercase letter, so "Register_Thread" should be "register_Thread", or even better "registerThread". Otherwise people looking at your code will assume that it is a class name. Same for "PPGresult".