Improve documentation of ANDROID_SDK_ROOT & ANDROID_HOME

[brodycj]

I think this is not so intuitive, see apache/cordova-android#922

I do think we could have solved it with only one of these environment variables defined but wanted to get the user's issue resolved asap.

Maybe should be done as part of the project in #1057.

[bkmgit]

ANDROID_SDK_ROOT seems to be the correct one currently, ANDROID_HOME seems to have been deprecated.

[breautek]

The current dev docs removed usages of ANDROID_HOME and uses ANDROID_SDK_ROOT instead. I think this issue is satisfied? If not, we should document what specifically needs to be improved.

[shawnurquhart]

Any chance you can please include a link to those specific docs thanks. I'm not sure on the actual official docs. Sorry I'm a bit new to this.

[breautek]

Any chance you can please include a link to those specific docs thanks. I'm not sure on the actual official docs. Sorry I'm a bit new to this.

The changes are not live (they will be with the docs for 10.x cordova cli is released), but you can view it the latest unreleased docs at https://github.com/apache/cordova-docs/blob/master/www/docs/en/dev/guide/platforms/android/index.md#setting-environment-variables