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

[gferon]

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.