Use DTO to extract placeholders

[eylulnc]

Description

Currently, the Android app processes course notifications using a legacy placeholder array mechanism. When a CourseNotificationDTO is received:

It is converted into a list of String placeholders.

These placeholders are later parsed to determine:
Notification content (e.g., title, body).
Navigation destination and parameters (e.g., course ID, post ID).

This indirect approach was introduced as a temporary compatibility solution but has now become a technical debt that hinders further development and maintainability.

Goal

Refactor the notification handling mechanism to use the CourseNotificationDTO and related DTOs directly, eliminating the placeholder array logic.

Hint: See iOS implementation