PointerChoreographer: Reset stylus icon on HOVER_EXIT

Unlike mouse and touchpad, a stylus pointer doesn't always move
"continuously" across the screen. It can jump from one place to another
by exiting hover at one location and entering hover in a different
place. When it first hovers in a new location, the same icon would have
been used in the new place as in the previous location until the app in
the new location specifies a new icon. If the app wants to show a
different icon, this can result in the old icon "flashing" for a short
period before it is updated.

To eliminate the flashing of the old icon, we will reset the icon on
HOVER_EXIT to the unspecified type.

Bug: 298074290
Test: manual
Test: atest inputflinger_tests
Change-Id: Ifdc7d999d39f2620b5ff91025b2d24ee2e23e4b9