## Description
This fixes some issues restoring the app state after it is killed by the
system, such as to free up memory while the screensaver is showing.
### Dev notes
Reproduce:
1. Start playing media from beginning
2. Pause at 10 minute mark
3. Wait for screensaver to start
4. Press a button
5. Media starts from beginning
Note: It may take a few iterations of 2-4 before the OS destroys the
app.
If the app is killed during playback, ideally Wholphin restores state to
_previous_ page, not playback since restoring playback is complex.
Before this PR, the back stack was maintained by Compose's
`rememberSerializable` and would be restored _after_ the
`PlaybackLifecycleObserver` cleans up the playback destination.
This meant that the app would be restored, from scratch, to the playback
page. The `PlaybackViewModel` would be initialized from scratch as it
was originally called, ie play from beginning.
The fix is to save and restore the back stack outside of Compose.
### Related issues
Fixes#767
### Testing
Emulator & nvidia shield
## Screenshots
N/A
## AI or LLM usage
None
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