Query input device for initial slot index.

This fixes a problem where touches can get stuck because the
driver and the framework have different ideas of what the
initial slot index is.  The framework assumed it was slot 0
but it could in principle be any slot, such as slot 1.  When
that happened, the framework would start tracking the first
touch as slot 0, but it might never receive an "up" for that slot.

Change-Id: Idaffc4534b275d66b9d4360987b28dc2d0f63218