Fixed crash on Windows Subsystem for Android, which doesn't have a USB manager.

Fixes https://github.com/libsdl-org/SDL/issues/5133

android-project/app/src/main/java/org/libsdl/app/HIDDeviceManager.java

@@ -141,6 +141,9 @@ public class HIDDeviceManager {
 
     private void initializeUSB() {
         mUsbManager = (UsbManager)mContext.getSystemService(Context.USB_SERVICE);
+        if (mUsbManager == null) {
+            return;
+        }
 
         /*
         // Logging