ANDROID: virtio: virtio_input: Set the amount of multitouch slots in virtio input

Virtio input was missing the the amount of multitouch
slots and kernel was filtering out ABS_MT_SLOT events
for a screen is touched in more than one place.

Bug: 143488374
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: I617099435af311e6b0ee127b76eafe13834ea8f8
(cherry picked from commit af5cf146e4adae1dda52b006a86bab9d26e49a0e)