[Story] Opening the application from various entry points

Description

As a user, when I open the application from a notification or from an external permalink, any previous backstack will be lost so that I can predict what will be the effect of pressing back.

First back press: go to the room list Second back press (Android only): background the application

The user story is created to explicitly define what is the expected behavior in this case. The current implementation of EXA and EXI may already be as expected, so there is maybe no change to do on the codebases.

External entry points are:

The main launcher icon. In this case, if the app is not already running in the background, the room list is displayed, and pressing back just put the app into background again. If the app is already running, it just move the app to foreground and the existing backstack is not lost.
A notification with a room target: a new backstack is created with room list + target room
A link clicked on an external app. In this case, the room list will always be in the stack and more screen may be added to the stack. See the other user stories of the Epic for more details.
A universal-link (Element Web URL from browser) should open the app

Acceptance criteria

Size estimate

None

Dependencies

None

Out of scope

Open questions

### Questions

Subtasks

### Android
- [ ] https://github.com/element-hq/element-x-android/issues/2651

### iOS
- [ ] https://github.com/element-hq/element-x-ios/issues/2641

### Rust

### Other

element-hq / element-meta