[bazel.build] Problem with /start/android-app

https://bazel.build/start/android-app is out-of-date. I only wrote down a couple of the issues I encountered, so there may be more:

- I couldn't just install the r25b NDK and get it to work; I had to download version 21 through Android Studio.
- `30.0.3` is the only build-tools version this'll work with to be able to find dx.jar. `33` doesn't have it.

(from a clean VM, it should be easy enough to spot all of the errors just by running through the instructions)