I just recently downloaded the Android SDK. Downloading all the packages in the SDK Manager will take way too much time. So I'm wondering whether I should deselect all the platforms except the latest one. Or do I need all the platforms? Thanks in advance.
Not at all. You just need to install those platforms which you are targeting for your apps and which you intend to test your app with. If I were you, I'd install:
You should also install
Optional: