Fix issues when the system kills the app during playback (#1116)

## 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

app/src/main/java/com/github/damontecres/wholphin/services/NavigationManager.kt

@@ -1,6 +1,5 @@
 package com.github.damontecres.wholphin.services
 
-import androidx.navigation3.runtime.NavKey
 import com.github.damontecres.wholphin.ui.nav.Destination
 import org.acra.ACRA
 import timber.log.Timber
@@ -14,7 +13,7 @@ import javax.inject.Singleton
 class NavigationManager
     @Inject
     constructor() {
-        var backStack: MutableList<NavKey> = mutableListOf()
+        var backStack: MutableList<Destination> = mutableListOf()
 
         /**
          * Go to the specified [com.github.damontecres.wholphin.ui.nav.Destination]