env/android-amd64-emu: add ANDROID_SDK_ROOT
env/android-amd64-emu: add ANDROID_SDK_ROOT

ANDROID_HOME is deprecated. golang.org/cl/244057 is replacing it
with ANDROID_SDK_ROOT, and we need the builder to use the new
environment variable. After the change is submitted and gomobile
is updated, we will need another clean up cl to delete ANDROID_HOME.

Updates golang/go#39945

Change-Id: I1f06cead94fbf5cd3c1ca1d8c6c68b2a219898eb
Reviewed-on: https://go-review.googlesource.com/c/build/+/247357
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build