Convert LooperExecutorTest in AppRTC Demo into JUnit test

LooperExecutorTest now uses Robolectric instead of being instrumentation
test. This allows the test to be run faster and easier.

Review-Url: https://codereview.webrtc.org/1989813002
Cr-Commit-Position: refs/heads/master@{#12825}

webrtc/examples/androidapp/src/org/appspot/apprtc/util/LooperExecutor.java

@@ -72,7 +72,7 @@ public class LooperExecutor extends Thread implements Executor {
     handler.post(new Runnable() {
       @Override
       public void run() {
-        Looper.myLooper().quit();
+        handler.getLooper().quit();
         Log.d(TAG, "Looper thread finished.");
       }
     });
@@ -131,4 +131,10 @@ public class LooperExecutor extends Thread implements Executor {
     }
   }
 
+  /**
+   * Access to the handler for testing purposes.
+   */
+  Handler getHandler() {
+    return handler;
+  }
 }