Backspace to clear selected shortcut key combination

After selecting a key combination, user can now press backspace to clear
the selected key combo.

Fix: 387995731
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Test: ShortcutCustomizationViewModelTest
Change-Id: Idcec42d9f2094ffb75915d75d6aa9f49033dc115