I plan to buy several phones which I will be sharing among different potential customers for demoing my app. The phones will have my app preinstalled and will be on my Google account, so I don't want my customers to be able to install other apps or buy stuff with the phone. How can I do that?
Android has APIs for dedicated devices that are owned by a company and dedicated for use for a specific purpose, such as serving as a kiosk device or a demo (like yours).
These APIs are designed to secure these fully managed devices such that they can be left unattended with untrusted users, which it sounds like applies to your example.