Disabling elements on the A11y Shortcut setting subpage

Elements on the subpage get disabled if there are no enabled shortcut targets for the relevant type.
In the case of A11y button, the items on its fragment become unsearchable when the setting is disabled.

Test: Manually verify conditions described above & in bug
Bug: 349180207
Flag: com.android.settings.accessibility.fix_a11y_settings_search
Change-Id: Id39e2eda6c9d1de4cdbfcbc22b8a1f443e2822d9