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/androidTest/java/com/github/damontecres/wholphin/test/InstrumentedBasicUiTests.kt

@@ -9,7 +9,9 @@ import androidx.compose.ui.test.onNodeWithTag
 import androidx.compose.ui.test.onNodeWithText
 import androidx.compose.ui.test.performKeyInput
 import androidx.compose.ui.test.pressKey
+import androidx.navigation3.runtime.NavBackStack
 import com.github.damontecres.wholphin.MainContent
+import com.github.damontecres.wholphin.services.NavigationManager
 import com.github.damontecres.wholphin.services.ScreensaverService
 import com.github.damontecres.wholphin.services.ScreensaverState
 import com.github.damontecres.wholphin.services.SetupDestination
@@ -43,16 +45,17 @@ class InstrumentedBasicUiTests {
     @OptIn(ExperimentalTestApi::class)
     @Test
     fun myTest() {
+        val navigationManager = NavigationManager()
+        navigationManager.backStack = NavBackStack(Destination.Home())
         // Start the app
         composeTestRule.setContent {
             WholphinTheme {
                 MainContent(
                     backStack = mutableListOf(SetupDestination.ServerList),
-                    navigationManager = mockk(relaxed = true),
+                    navigationManager = navigationManager,
                     appPreferences = mockk(relaxed = true),
                     backdropService = mockk(relaxed = true),
                     screensaverService = screensaverService,
-                    requestedDestination = Destination.Home(),
                     modifier = Modifier,
                 )
             }