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
diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index 289b5de..41dd11e 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -184,7 +184,9 @@
 
 Future<void> _flutterBuildApk(String relativePathToApplication) async {
   // TODO(dnfield): See if we can get Android SDK on all Cirrus platforms.
-  if (Platform.environment['ANDROID_HOME']?.isEmpty ?? true) {
+  if (
+        (Platform.environment['ANDROID_HOME']?.isEmpty ?? true) &&
+        (Platform.environment['ANDROID_SDK_ROOT']?.isEmpty ?? true)) {
     return;
   }
   print('Running APK build tests...');