fix: keep deck selection dialog open for multi-select in widget config

[pankaj0695]

Purpose / Description

Resolves the TODOs in

Anki-Android/AnkiDroid/src/main/java/com/ichi2/widget/deckpicker/DeckPickerWidgetConfig.kt

Lines 157 to 158 in 5fbbe83

	// TODO: Implement multi-select functionality so that user can select desired decks in once.
	// TODO: Implement a functionality to hide already selected deck.

and the related TODO at

Anki-Android/AnkiDroid/src/main/java/com/ichi2/widget/deckpicker/DeckPickerWidgetConfig.kt

Line 341 in e6104eb

// TODO: Eventually, ensure that the user can't select a deck that is already selected.

• Users had to repeatedly open/close the deck selection dialog to add multiple
  decks to the widget
• Already selected decks were still visible in the dialog, leading to a "already selected" error

Approach

• Added a keepOpen parameter (default false) to DeckSelectionDialog.newInstance() so existing callers are unaffected
• When keepOpen is true, selecting a deck notifies the listener and removes the deck from the dialog list instead of dismissing
• The dialog auto-dismisses when the list is empty or the 5-deck limit is reached
• DeckPickerWidgetConfig now filters out already-selected decks before showing the dialog and passes keepOpen = true

How Has This Been Tested?

• Existing DeckSelectionDialogTest - 2/2 tests passing
• Manual testing on a physical Android 15 device:
  • Verified dialog stays open after each deck selection
  • Verified selected decks are removed from the dialog list
  • Verified dialog auto-closes on reaching the 5-deck limit
  • Verified previously selected decks reappear in the dialog after removal from widget config
  • Verified Cancel/back button dismisses dialog correctly

Before

Before.mp4

After

After.mp4

Checklist

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• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner