Exorcise notification panel ghosts.

Bug: 5105599

Don't clear slot data when slot is no longer in use.

The kernel will not resend absolute slot values if they are
unchanged, even if the slot was previously marked unused.

Because the protocol is stateful, this could cause problems if
evdev drops events (marked by SYN_DROPPED) since we cannot
query the initial values of the slots, but there's nothing
we can do.

Change-Id: Ie13e68097fb8ed2542c2f60338b499082cf9e77f