I did a quick search on "Arduino App Inventor Bluetooth app". The examples I found were more complicate than you need.
Take a look at this one:
BT app example
You could also search the Arduino forum as other people have discussed Arduino,BT & apps.
Your main task would be to learn MIT App Inventor. The app itself for your needs would be quite simple. That said, you still need to learn App Inventor.
Maybe follow Nick's & cattledog's advice first might be the best way to go.