feat: support folder selection from other apps via Storage Access Framework

[alvaroemtnez]

Summary

OpenCloud has a correct implementation to select its locations from the system files app via the Storage Access Framework. However, as stated in #99 and #45, selecting a folder from an app was not supported, as the ACTION_OPEN_DOCUMENT_TREE intent of the Android Storage Access Framework (SAF) needed to be implemented.

This PR adds support for that intent, and also, as some apps filter the SAF locations and allow only local storage (which OpenCloud can be, if a folder is selected as "available offline", so filtering it makes no sense but hardcoding it as local storage doesn't either), adds a toggle in the Advanced section of settings to pretend to be so, by adding the FLAG_LOCAL_ONLY flag, as other apps like RSAF do, and the corresponding translations for the settings item.

As granting an app access to a location in OpenCloud can mean that the app in question does multiple concurrent operations, light changes have been made, replacing fileToUpload singleton with ConcurrentHashMap and UUIDs to support multi-threaded file creation.

Code changes

• Added the isChildDocument function override to opencloudApp/src/main/java/eu/opencloud/android/presentation/documentsprovider/DocumentsStorageProvider.kt to implement the ACTION_OPEN_DOCUMENT_TREE intent.
• Also in opencloudApp/src/main/java/eu/opencloud/android/presentation/documentsprovider/DocumentsStorageProvider.kt, minor changes to use ConcurrentHashMap and random UUIDs for a more robust upload queue.
• Added support in the addRoot function in opencloudApp/src/main/java/eu/opencloud/android/presentation/documentsprovider/cursors/RootCursor.kt to pass a boolean to enable pretending to be local storage.
• Added a settings option in the Advanced menu to pretend to be local storage (modified opencloudApp/src/main/res/xml/settings_advanced.xml and opencloudApp/src/main/java/eu/opencloud/android/presentation/settings/advanced/SettingsAdvancedFragment.kt)
• Added translations for the advanced menu settings item for all supported languages. A subtitle warns the user of the possible unpredictable behaviour pretending to be local storage could originate in slow network connections.

Limitations

• The URI displayed to the user (if the app in question displays an URI) can be counter-intuitive, as it will be similar to content://eu.opencloud.documents/tree/[ID], where [ID] is an integer defining the directory or file in the internal app tree. As far as I know, displaying the full URI that the user sees in their cloud web UI would require deep and breaking changes (which would probably not be worth, at the moment).
• Concurrent operations are supported with the UUID based queue (which didn't require deep changes), however, if an app requires thousands of concurrent file operations (i.e., Signal backups, that concurrently read, write, delete, move, etc. files, which is very intensive), that can lead to unexpected behaviour (upload errors, or the entire files directory that signal uses replaced by an empty version). Signal was the only use case I tested where this happened, and only in the very recent new backup system, which is designed to be local only. These use cases seem out of scope for this PR, and would require a regular folder sync like in the desktop app, or a straight WebDAV mount (which RSAF and others already do), which wouldn't be appropriate to implement only to support one edge case. SeedVault could also maybe create the same problems, but again, extremely operation intensive cases seem out of scope when there are solutions specifically tailored for this, and those backups were meant to be done locally, not in a network mount.

AI use

LLMs (Gemini) were used for assistance in code generation and for the translations. All generated code was reviewed and tested and can be easily audited.