Improve thumbnail sharing

Since vlc-android 3.3.x, we changed the way we shared thumbnail, mostly by using any file we find in an analyzed folder and assign it as the album cover. This is not good enough, and is causing incorrect results when users are storing all their trakcs in the same folder.

In order to improve this, we need to:

• Prioritize embedded artworks over side image files
• Implement hashing to be able to share embedded thumbnails if they are identical
• Keep using whatever we can find in the folder in case nothing else is found attached to the media
• Implement a resize if the cover are too large, we don't need 4K cover in most cases.

The hash doesn't need to be something complicated. The file size + the first N bytes might be enough, we don't care much about collisions, but we care about the result being fast.