[Architecture] Allow Activity to be injected through the dagger graph.

We might need to inject an activity context in some of our classes used by Presenter or others.
We can trick it by using the `LocalContext.current` but it's a hack.
We should probably create a `Component` owned by `MainActivity` so we can inject it in subcomponents.
It could require to create new subcomponents for Session and Room, as the existant ones can be used outside the context of an activity (like from notification).