Fix Previous episode button showing on the first episode

adjacentTo returns [current, next] on the first episode, and the old code took the first sibling that was not the current item, so it picked the next episode as the previous one. Only count an item that sits before the current one.

app/src/main/java/org/jellyfin/androidtv/ui/itemdetail/FullDetailsFragmentHelper.kt

@@ -217,10 +217,8 @@ fun FullDetailsFragment.populatePreviousButton() {
 			).content
 		}
 
-		val previousItem = siblings.items
-			.filterNot { it.id == mBaseItem.id }
-			.firstOrNull()
-			?.id
+		val currentIndex = siblings.items.indexOfFirst { it.id == mBaseItem.id }
+		val previousItem = siblings.items.getOrNull(currentIndex - 1)?.id
 
 		mPrevItemId = previousItem
 		mPrevButton.isVisible = previousItem != null