Prefer getAdbPath(androidSdk) over androidSdk.adbPath (#4076)

For developers with the platform tools installed but no SDK, it prevents
crashing in the flutter tool in favor of more user-friendly error messages
downstream.
diff --git a/packages/flutter_tools/lib/src/android/android_sdk.dart b/packages/flutter_tools/lib/src/android/android_sdk.dart
index d16a4cf..3ac5b21 100644
--- a/packages/flutter_tools/lib/src/android/android_sdk.dart
+++ b/packages/flutter_tools/lib/src/android/android_sdk.dart
@@ -29,6 +29,9 @@
 };
 
 /// Locate ADB. Prefer to use one from an Android SDK, if we can locate that.
+/// This should be used over accessing androidSdk.adbPath directly because it
+/// will work for those users who have Android Platform Tools installed but
+/// not the full SDK.
 String getAdbPath([AndroidSdk existingSdk]) {
   if (existingSdk?.adbPath != null)
     return existingSdk.adbPath;