Early 2009, or Late 2009? How large a monitor?
I had the base model Early 2009 imac. Last time I checked--about a year ago-- it was still usable (with 8GB) , but most publishers and Apple just assume you have a faster machine. I never got the courage to disassemble it and put in an SSD, which might have delayed final obsolescence for some time to come, but the nVidia 9400 video chipset is essentially obsolete, and the Core 2 Duo doesn't have the nice video compression support that more modern CPUs have.
It also lacks USB3, and firewire 800 devices are rare and expensive.
Frankly, a mac mini of considerably more recent vintage might be a better option. The built in screen on a Early 2009 20 inch model isn't IPS, so it's no great asset.
I don't develop for android, but I would imagine that it allows for code to be run on a simulated android device. An imac of that vintage is probably slower than a great real android devices. That might prove to be a source of frustration.