[Task] Unify the way we decide when a room is actually a DM

Some time ago we took a decision to enforce DMs being only "direct rooms with at most 2 users, encrypted".

As we developed the app, we apparently duplicated the computation of this value in several places ([here](https://github.com/element-hq/element-x-android/blob/010a90f9ffb0db9411cbfc69bf8fd5f87b029e7d/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/roomlist/RoomSummaryDetailsFactory.kt#L50), [here](https://github.com/element-hq/element-x-android/blob/010a90f9ffb0db9411cbfc69bf8fd5f87b029e7d/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/room/MatrixRoom.kt#L60), [here](https://github.com/element-hq/element-x-android/blob/010a90f9ffb0db9411cbfc69bf8fd5f87b029e7d/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/analytics/JoinedRoomExt.kt#L35)... and in other many places, with different outputs) so we should try to ensure consistency.