Generating a wallet on a android device is not safe to start with but I won't go into that as its up to you.
Yep, this is a pretty big issue since your mobile phone is usually more connected to the internet to your other devices (laptop, desktop computer, eg). I'd recommend using GreenAddress online compared to just using your phone if you have a laptop/PC.
If you're so tight about security, wouldn't it just be smarter to get a ledger wallet? They are 100 percent safer then greenaddress, and the private keys would never enter your phone, and they added a ledger app for mobile a decent time ago.