Implement proper Opml import - and do some cleanup in the SubscriptioptionManager

[gzsombor]

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[Copilot]

Pull request overview

This PR implements proper OPML import functionality for YouTube subscriptions and refactors the SubscriptionsBackupsManager to improve code organization and maintainability.

Changes:

• Added a dedicated OpmlParser class with comprehensive test coverage for parsing OPML files
• Refactored SubscriptionsBackupsManager to extract duplicate callback logic and simplify threading management
• Added UI elements and strings to support OPML import functionality in the backup preferences

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
OpmlParser.java	New parser implementation for OPML files with support for various YouTube URL formats
OpmlParserTest.java	Comprehensive test suite for OPML parser covering basic parsing, folders, parameters, and edge cases
SubscriptionsBackupsManager.java	Refactored to use new OpmlParser, extracted UriReceivedCallback class, simplified threading logic
OpmlExporterTest.java	Added test to verify exported OPML can be parsed back
OpmlExporter.java	Removed unused imports
preference_backup.xml	Added OPML import preference UI element
strings_preferences.xml	Added strings for OPML import feature
strings_subs.xml	Updated error message text
BackupPreferenceFragment.java	Added click handler for OPML import preference
app/build.gradle	Added XML Pull Parser test dependency
basic.opml, empty.opml, missing_attrs.opml, user_urls.opml, with_folders.opml, with_params.opml	Test resource files for OPML parsing

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