Add helpful examples to Android test env validation failure messages.

Summary:
Hi there!

I set up the repo because I want to do a little contribution to the Android side but ran into trouble [running tests](https://facebook.github.io/react-native/docs/contributing.html). The Android environment validation kept failing. While the messages were a little helpful, it would have saved me a bit of time and research if I had some helpful examples to copy-and-paste. In my case, I'm using Android SDK Tools on the command line. Hopefully, this will help others when setting up!

Run `./scripts/run-android-local-unit-tests.sh` with each of the following to see the expanded help messages:

* Make sure `$ANDROID_HOME/platforms/android-$MAJOR` is _not_ present.
* Make sure `$ANDROID_HOME/build-tools/$BUILD_TOOLS_VERSION` is _not_ present.
* Make sure `JAVA_HOME` is _not_ set and/or is _not_ in `PATH`.
Closes https://github.com/facebook/react-native/pull/16222

Differential Revision: D6044875

Pulled By: ericnakagawa

fbshipit-source-id: febacbd08fb632b349c352035f24eed891fbd154