Hello, For me personally, I would want isolation(optical, transformer, etc) between car and VIM3 for measuring car system voltage. GPIO offers ADC on pins 10 and 12. Devices(search term: voltage detection module) made for Arduino or Pi using ADC(Analog-To-Digital-Converter) are available, look like a very simple circuit. But those seem to lack isolation. Also, how you would make it useful for Android is not known to me.
Another option would be to use an OBDII Bluetooth device to send an android app voltage and most other relative automotive data. User @tschultz made a dashboard display for his truck. Maybe you can get some ideas reviewing that thread.
Also user @davemf has done some automotive projects, clicking his name will take you to a page with his posts and projects.
Hope it helps.