If you want instead to remain with Google, you can distribute the app with your API Key and limit the usage to remain inside the free monthly quota.
What I have stated is that as you open the app and load the map, Google counts one call to the map API. Then you can zoom, pan as you want with no additional counts.
Play with the console to verify this.
I dont' how many user are you pointing to, but if you limit the usage for example to 50 calls per day, you can have 50 user that open the app per day. Maybe it is enough.
The only little unpleasant aspect is that you must anyway create an account and give Google your billing information with credit card also.
But as you stay below the free quota you will receive invoice from Google with 0 to pay.