Clear out context in UsbTransferProvider.cancel()

Author: JulianKast

android/sdl_android/src/main/java/com/smartdevicelink/transport/UsbTransferProvider.java

@@ -179,6 +179,7 @@ public void checkIsConnected() {
     }
 
     public void cancel() {
+        context = null;
         if (isBound) {
             unBindFromService();
         }