[fenix] Issue https://github.com/mozilla-mobile/fenix/issues/4513: added isLoading to bookmark state

This was added to the state object as a top level param because it could reasonably coexist with any value of `tree` or `mode`. Even if we don't now, we may someday want to display a loading indicator while also showing cached bookmarks.

For now, we set isLoading to false whenever we receive any bookmarks

app/src/main/java/org/mozilla/fenix/library/bookmarks/BookmarkFragment.kt

@@ -73,7 +73,7 @@ class BookmarkFragment : LibraryPageFragment<BookmarkNode>(), BackHandler, Accou
         val view = inflater.inflate(R.layout.fragment_bookmark, container, false)
 
         bookmarkStore = StoreProvider.get(this) {
-            BookmarkFragmentStore(BookmarkFragmentState(null))
+            BookmarkFragmentStore(BookmarkFragmentState(null, isLoading = true))
         }
         bookmarkInteractor = BookmarkFragmentInteractor(
             bookmarkStore = bookmarkStore,

app/src/main/java/org/mozilla/fenix/library/bookmarks/BookmarkFragmentStore.kt

@@ -20,7 +20,11 @@ class BookmarkFragmentStore(
  * @property tree The current tree of bookmarks, if one is loaded
  * @property mode The current bookmark multi-selection mode
  */
-data class BookmarkFragmentState(val tree: BookmarkNode?, val mode: Mode = Mode.Normal) : State {
+data class BookmarkFragmentState(
+    val tree: BookmarkNode?,
+    val mode: Mode = Mode.Normal,
+    val isLoading: Boolean
+) : State {
     sealed class Mode {
         open val selectedItems = emptySet<BookmarkNode>()
 
@@ -58,7 +62,8 @@ private fun bookmarkFragmentStateReducer(
                     BookmarkFragmentState.Mode.Normal
                 } else {
                     BookmarkFragmentState.Mode.Selecting(items.toSet())
-                }
+                },
+                isLoading = false
             )
         }
         is BookmarkFragmentAction.Select ->