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)

(cherry picked from commit 328354f87874ece0ee3b0e6cd35e82b007e51b3c)

platform/android/export/export.cpp

@@ -1717,7 +1717,7 @@ 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;