fix: make variable volatile which is used in more than one thread

Author: marcnause

Diff for: app/src/main/java/io/pslab/sensors/AbstractSensorActivity.java

@@ -258,7 +258,7 @@ public boolean onOptionsItemSelected(MenuItem item) {
 
     protected abstract class SensorDataFetch {
 
-        private boolean isSensorDataAcquired;
+        private volatile boolean isSensorDataAcquired;
 
         protected float getTimeElapsed() {
             return (System.currentTimeMillis() - getStartTime()) / 1000f;