Move Matrix sessions management in a dedicated class

Move code in LegacyAppDelegate related to Matrix sessions management to a dedicated class. The idea is then to use this class in AppCoordinator in order to know when the navigation/view hierarchy should change and which sessions should be to injected to other coordinators.

Start by reading LegacyAppDelegate and locating the code responsible for Matrix sessions management, then inspect AppCoordinator to understand how navigation and session injection should use the extracted class. Done means that session management is handled by a dedicated class and AppCoordinator can determine navigation/view-hierarchy changes and the sessions passed to other coordinators.

Written by the indexing model from the issue text.