Full-text search

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Add full-text search so that contents of files is searchable

### Motivation

Even though this feature request is quite self-explanatory, I'm fully aware that this is an enormous development task. I just wanted to put it out there because I've received a feature request via email and I noticed that it was mentioned in a comment in the past https://github.com/cryptomator/android/issues/94#issuecomment-479498145.

Issue #94 could be a prerequisite because the underlying implementations of a recursive search over filenames could be useful for a full-text search index.

### Considered Alternatives

I'm not sure what a Document Provider might bring to the table. Maybe we don't even need to implement our own full-text search index.

### Anything else?

_No response_