Fallback to default app storage on Android when custom storage lacks permissions

[GooGuJiang]

Motivation

• Realm startup could fail on Android when using a custom storage path due to permission or read-only filesystem errors, causing unnecessary corrupt backups or startup failure.
• Provide a safer recovery path by falling back to the default app storage when permission failures are detected for custom storage on Android.

Description

• Added a using alias RuntimeInfo = osu.Framework.RuntimeInfo and a new fallback flow in the realm startup catch block which attempts tryFallbackToDefaultStorage before performing other recovery logic and backups.
• Implemented tryFallbackToDefaultStorage(Exception) to detect Android runtime, confirm usage of IO.OsuStorage with a custom path, inspect the root exception message for permission-related failures, log the event, reset the custom storage path, and return true to retry opening the realm.
• Added helper isStoragePermissionFailure(string) to centralize detection of permission/read-only errors in exception messages.

Testing

• Ran the repository unit test suite with dotnet test and observed no regressions in automated tests.
• Executed an Android startup integration check that simulates storage permission failures to verify the fallback path logs the event and retries with the default storage, which succeeded.

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Codex Task