Allows downloading the images sent as destructive pics

[Aeres-u99]

Steps to reproduce:

1. Two different devices A and B
2. Send media from A to B
3. Once the Media is received on B donot open it, instead turnoff wifi/data
4. Open the media and quickly navigate to /sdcard/Android/org.thunderdog.challegram/photos
5. Copy paste the image to any desired location.

Problem: Have noticed that the images sent using the timer can be saved and copied to anywhere on device without needing any root or specific methods. This method only requires people to turn off wifi and then it will be automatically downloaded
Solution: Instead of downloading/caching the images in /sdcard/Android/org.thunderdog.challegram/photos it should be done in /data/data/org.thunderdog.challegram/cache folder which is inaccessible to non root users.

Video:
https://imgur.com/a/MXiXkP7