Add collection extensions related to deck selection

One method to retrieve the 'Default' deck which is always available.
One method to retrieve the current selected deck if it's not a
filtered deck in which case return the 'Default' deck.

Author: lukstbit

AnkiDroid/src/main/java/com/ichi2/anki/DeckSpinnerSelection.kt

@@ -18,6 +18,8 @@ package com.ichi2.anki
 import androidx.fragment.app.Fragment
 import com.ichi2.anki.CollectionManager.withCol
 import com.ichi2.anki.dialogs.DeckSelectionDialog
+import com.ichi2.anki.libanki.Collection
+import com.ichi2.anki.libanki.Deck
 import com.ichi2.anki.libanki.DeckId
 import com.ichi2.anki.model.SelectableDeck
 
@@ -33,6 +35,18 @@ import com.ichi2.anki.model.SelectableDeck
  */
 const val ALL_DECKS_ID = 0L
 
+/**
+ * Returns the current selected deck only if it's not filtered otherwise returns the 'Default' deck.
+ */
+fun Collection.selectedDeckIfNotFiltered(): Deck {
+    val selectedDeck = decks.getLegacy(decks.selected())
+    return if (selectedDeck == null || selectedDeck.isFiltered) {
+        decks.getDefault()
+    } else {
+        selectedDeck
+    }
+}
+
 /**
  * Displays a [DeckSelectionDialog] for the user to select a deck, with the list of displayed decks
  * filtered based on the parameters of this method.

libanki/src/main/java/com/ichi2/anki/libanki/Decks.kt

@@ -693,6 +693,12 @@ class Decks(
     @NotInLibAnki
     fun isEmpty(did: DeckId): Boolean = cardCount(did) == 0
 
+    /**
+     * Retrieve the 'Default' deck which is always available.
+     */
+    @NotInLibAnki
+    fun getDefault(): Deck = getLegacy(Consts.DEFAULT_DECK_ID)!!
+
     companion object {
         // Parents/children