Clears the default service from A11Y_SHORTCUT B&R.

If the restored value contains the default service then it is removed,
unless the previous value (pre-restore) also contains the default
service.

Bug: 341374402
Flag: com.android.server.accessibility.clear_default_from_a11y_shortcut_target_service_restore
Test: atest AccessibilityManagerServiceTest
Test: Create a backup with the default service in the Setting.
      - Restore backup when the Setting is empty.
        Observe Setting is unchanged.
      - Restore backup when the Setting has another service.
	Observe Setting is unchanged.
      - Restore backup when the Setting already has the default service.
        Observe Setting keeps the default service.
Change-Id: Id7f9e6de96f032abf1f073cbef801fd2dbcee9a1