Fix android-sdk caveats

Android sdk location for IDEs should be `opt_prefix` or you'll have to change the path every time you update the sdk.
I believe this is a mistake from the commit below, in which he could have missed one of `#{prefix}`s.
https://github.com/Homebrew/homebrew/commit/276fd63718772f921a2330fc36619c7e474c0336

@adamv, please check if you can. Thanks