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/WholphinDreamService.kt

@@ -22,9 +22,7 @@ import androidx.savedstate.SavedStateRegistryOwner
 import androidx.savedstate.setViewTreeSavedStateRegistryOwner
 import com.github.damontecres.wholphin.data.ServerRepository
 import com.github.damontecres.wholphin.preferences.AppPreferences
-import com.github.damontecres.wholphin.preferences.UserPreferences
 import com.github.damontecres.wholphin.services.ScreensaverService
-import com.github.damontecres.wholphin.services.UserPreferencesService
 import com.github.damontecres.wholphin.ui.components.AppScreensaverContent
 import com.github.damontecres.wholphin.ui.launchDefault
 import com.github.damontecres.wholphin.ui.theme.WholphinTheme
@@ -33,6 +31,7 @@ import dagger.hilt.android.AndroidEntryPoint
 import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.flow.first
 import org.jellyfin.sdk.model.serializer.toUUIDOrNull
+import timber.log.Timber
 import javax.inject.Inject
 import kotlin.time.Duration.Companion.milliseconds
 
@@ -61,6 +60,7 @@ class WholphinDreamService :
 
     override fun onCreate() {
         super.onCreate()
+        Timber.d("onCreate")
 
         savedStateRegistryController.performRestore(null)
         lifecycleRegistry.currentState = Lifecycle.State.CREATED
@@ -74,6 +74,7 @@ class WholphinDreamService :
 
     override fun onAttachedToWindow() {
         super.onAttachedToWindow()
+        Timber.d("onAttachedToWindow")
         val itemFlow = screensaverService.createItemFlow(lifecycleScope)
         setContentView(
             ComposeView(this).apply {
@@ -109,11 +110,13 @@ class WholphinDreamService :
 
     override fun onDreamingStarted() {
         super.onDreamingStarted()
+        Timber.d("onDreamingStarted")
         lifecycleRegistry.currentState = Lifecycle.State.STARTED
     }
 
     override fun onDreamingStopped() {
         super.onDreamingStopped()
+        Timber.d("onDreamingStopped")
         lifecycleRegistry.currentState = Lifecycle.State.DESTROYED
     }
 }