chore(ui): drain Handler thread queue when activity is destroyed (#99)

Currently, we don't have any tasks that could execute after the Activity
is destroyed and lead to crashes or unexpected behaviors. So, it's
better to drain this static Handler thread's queue upon Activity
destruction to prevent quite realistic, well-known bugs tied to Android
lifecycle that we might face in the future if making any changes.

Author: iusmac

src/com/github/iusmac/sevensim/ui/scheduler/SchedulerActivity.java

@@ -160,4 +160,11 @@ protected void onPause() {
         mSubscriptions.removeOnSubscriptionsChangedListener(this);
         sHandler.removeCallbacksAndMessages(mSubscriptionsChangedToken);
     }
+
+    @Override
+    protected void onDestroy() {
+        super.onDestroy();
+
+        sHandler.removeCallbacksAndMessages(null);
+    }
 }

src/com/github/iusmac/sevensim/ui/sim/SimListActivity.java

@@ -145,6 +145,13 @@ protected void onPause() {
         unregisterReceiver(mIntentReceiver);
     }
 
+    @Override
+    protected void onDestroy() {
+        super.onDestroy();
+
+        sHandler.removeCallbacksAndMessages(null);
+    }
+
     @VisibleForTesting
     final class IntentReceiver extends BroadcastReceiver {
         @Override