android: pressing "Back" on the app's main screen does not exit the app

[Steffo99]

Describe the bug

Pressing "Back" at the main screen of an Android app typically exits it.

Sable does not currently follow that convention.

Reproduction

1. Open Sable Android
2. Open a chat
3. Press "Back"
4. You should be back at the chat list
5. Press "Back"
6. Nothing happens

Expected behavior

The activity should quit, either immediately, or after back is pressed again.

Platform and versions

1. OS: Android 13
2. Platform: Tauri
3. Sable Version: v1.21.0 (stable)

Additional context

(Can't try with Nightly right now.)

[Steffo99]

cc @SnowyCoder