Run sensor "change handler" also with the initial value

libs/core/input.ts

@@ -13,6 +13,7 @@ namespace sensors.internal {
 
         control.runInBackground(() => {
             let prev = query()
+            changeHandler(prev, prev)
             while (true) {
                 loops.pause(periodMs)
                 let curr = query()

libs/core/linux.cpp

@@ -397,6 +397,10 @@ static void runPoller(Thread *thr) {
     // note that this is run without the user mutex held - it should not modify any state!
     TValue prev = pxt::runAction0(query);
 
+    startUser();
+    pxt::runAction2(thr->act, prev, prev);
+    stopUser();
+
     while (true) {
         sleep_core_us(us);
         if (paniced)