Android: Upgrade SDL and SDL_ttf

- SDL 2.0.20
- SDL_ttf 2.0.18

android/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

android/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -1479,6 +1479,19 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
         return mLastCursorID;
     }
 
+    /**
+     * This method is called by SDL using JNI.
+     */
+    public static void destroyCustomCursor(int cursorID) {
+        if (Build.VERSION.SDK_INT >= 24) {
+            try {
+                mCursors.remove(cursorID);
+            } catch (Exception e) {
+            }
+        }
+        return;
+    }
+
     /**
      * This method is called by SDL using JNI.
      */