You could run the App in an instance of the Android emulator, a VM which is part of the free Android SDK.
Running the emulator in a regular VNC server session, rather than a VNC server App running on the emulator, would most likely provide better performance.
The VNC session and the Android emulator session could be started as needed from a web page with an embedded VNC viewer Java applet. On Ubuntu, for example, this is Java viewer client is freely available in the vnc-java package.
To avoid inadvertent data sharing between users, it would be necessary to run each VM session with a clean copy of the Android VM's AVD and virtual SD-card instance.
To reduce the VM copy and start-up time for the emulator sessions, it may be desirable to have the web server maintain a few active sessions started and ready for use. This would provide clean individual sessions for your potential customers to try.