Fix detection logic for the Android sdk path

The previous logic used the 'tools' directory within the Android sdk to validate it. That directory was recently deprecated and removed from the Android sdk folder (https://developer.android.com/studio/releases/sdk-tools)

platform/android/export/export.cpp

@@ -1699,7 +1699,7 @@ public:
 				valid = false;
 			} else {
 				Error errn;
-				DirAccessRef da = DirAccess::open(sdk_path.plus_file("tools"), &errn);
+				DirAccessRef da = DirAccess::open(sdk_path.plus_file("platform-tools"), &errn);
 				if (errn != OK) {
 					err += TTR("Invalid Android SDK path for custom build in Editor Settings.") + "\n";
 					valid = false;