Ensure we have the Java binary on PATH when invoking avdmanager (#18908)

See https://github.com/flutter/flutter/issues/13379#issuecomment-400984667.
diff --git a/packages/flutter_tools/lib/src/android/android_sdk.dart b/packages/flutter_tools/lib/src/android/android_sdk.dart
index 2dcc5bf..d547cef 100644
--- a/packages/flutter_tools/lib/src/android/android_sdk.dart
+++ b/packages/flutter_tools/lib/src/android/android_sdk.dart
@@ -466,6 +466,8 @@
   }
 
   Map<String, String> _sdkManagerEnv;
+  /// Returns an environment with the Java folder added to PATH for use in calling
+  /// Java-based Android SDK commands such as sdkmanager and avdmanager.
   Map<String, String> get sdkManagerEnv {
     if (_sdkManagerEnv == null) {
       // If we can locate Java, then add it to the path used to run the Android SDK manager.