Proposal: use a path relative to `ANDROID_HOME` like in Android Gradle Plugins

I'm building an app with Bazel and `flutter` (which sadly is not integrated at all) and was puzzled when I realized `ANDROID_NDK_HOME` is completely ignored by the latest version of the Android Gradle plugin.

It looks like the project has moved from using `ANDROID_NDK_HOME` to assuming relative paths within `ANDROID_HOME`, like so: https://github.com/android/ndk-samples/wiki/Configure-NDK-Path#introduction

Adding search paths like `$ANDROID_HOME/ndk/$version` would also let us specify a version in the `repository_rule` which would fail if the constraint is not satisfied on the host's machine.