Support ANDROID_SDK_ROOT in addition to ANDROID_HOME (#25221)

* Fall back to ANDROID_SDK_ROOT if ANDROID_HOME is not set

And update descriptions to use the non-deprecated ANDROID_SDK_ROOT.

Fixes #15114.

* Remove trailing whitespace

* Update dev/devicelab/lib/framework/adb.dart

Co-Authored-By: DanTup <danny@tuppeny.com>

* Reformat long line