Android build: use ANDROID_NDK_HOME rather than ANDROID_NDK

It apepars that ANDROID_NDK_HOME is the recommended standard
environment variable for the NDK.

We retain ANDROID_NDK as a fallback.

Fixes #8101

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8103)