android: Implement SAF support & migrate to SDK 31. (#4)

src/common/fs/path_util.h

@@ -180,6 +180,14 @@ template <typename Path>
 }
 #endif
 
+/**
+ * Sets the directory used for application storage. Used on Android where we do not know internal
+ * storage until informed by the frontend.
+ *
+ * @param app_directory Directory to use for application storage.
+ */
+void SetAppDirectory(const std::string& app_directory);
+
 /**
  * Gets the filesystem path associated with the YuzuPath enum.
  *