Heads-Up Notification: Update touchable region correctly.

This assumes HUN is always at the top (y-position = 0), but on Desktop
devices this is not the case since HUN is at the bottom. Update the
y-position accordingly.

Test: Tapping on the HUN works again
Bug: 221275739
Change-Id: Ic31ecc0f3151f9749e91f720f97afc8a85cb2eb6