I see you have added approx. location to the permissions of your Android app.
IMO, next time you are going to add some permissions, you should consider also adding GET_ACCOUNTS and USE_CREDENTIALS.
I really don't like entering username and password (I can never remember my password for one thing), and I use Fongo on my phone and my tablet. Sometimes I have to re-enter my Fongo credentials and I'm not even sure why.
So, I really like the trend towards authenticating users via their Google account. This does not mean changing Fongo account system at all - you can create an equivelency between their google account and their Fongo account on your server. The new Play store API's make this much easier for everyone - actually, it is now invisible for the end-user.
The hardest part is introducing new permissions to your users - which I why I suggest you look into this the next time you have some other permission you want to add.
Tom