Ensure getNotificationKeysForItem() returns empty on unsupported items.

We were bypassing the check in getBadgeInfoForItem(), which meant
we would return notifications based on package/user regardless of
the item type. In particular, deep shortcuts would show notifications
when long-pressed.

Bug: 34866646
Change-Id: I6575a28539313c0c5d0eea1cbf7b7726f747bcd5