Cache metadata directory

In the Android application, when opening a folder that has a lot of long file names, the app needs to open each .lng file and traverse to the corresponding metadata file to find the full name. This can be extremely slow with folders of hundreds of files.

Given that metadata names are static and based on the SHA1 where collisions are unlikely, it should be possible to cache the entire m/ directory.

In fact, it looks like Cryptomator desktop apps don't clean up the SHA1 hashes at all.

Proposed solutions:

1. Cache the m/ directory. This should not really take much space given this is really just a big hashtable of SHA1 to filename
2. Have an alternate thread that preloads the entire m/ directory for faster traversals in the future.