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/packages/flutter_tools/lib/src/android/android_sdk.dart b/packages/flutter_tools/lib/src/android/android_sdk.dart
index 2f7ffdb..7ffd46f 100644
--- a/packages/flutter_tools/lib/src/android/android_sdk.dart
+++ b/packages/flutter_tools/lib/src/android/android_sdk.dart
@@ -21,6 +21,7 @@
 AndroidSdk get androidSdk => context[AndroidSdk];
 
 const String kAndroidHome = 'ANDROID_HOME';
+const String kAndroidSdkRoot = 'ANDROID_SDK_ROOT';
 
 // Android SDK layout:
 
@@ -243,6 +244,8 @@
         androidHomeDir = config.getValue('android-sdk');
       } else if (platform.environment.containsKey(kAndroidHome)) {
         androidHomeDir = platform.environment[kAndroidHome];
+      } else if (platform.environment.containsKey(kAndroidSdkRoot)) {
+        androidHomeDir = platform.environment[kAndroidSdkRoot];
       } else if (platform.isLinux) {
         if (homeDirPath != null)
           androidHomeDir = fs.path.join(homeDirPath, 'Android', 'Sdk');