Optimizing OCFileListAdapter's Filtering Mechanism

**Problems:**

1. Many unused functions exist in OCFileListAdapter.
2. Filtering the mFiles variable multiple times causes performance issues and code repetition.

**Solution:**

1. Delete unused functions.
2. Use **java.util.function.Predicate** instead of repeating logic for filtering, such as in filterHiddenFiles(), filterByMimeType(), limitToPersonalFiles(), etc.