ion: add new ion_user_handle_t type for the user-space token

Declare new ion_user_handle_t type to contain the token returned to user-space.
This allows a 2-step migration of the user-space code to a new kernel header
first, then will allow us to change the definition of the ion_user_handle_type_t
to int without breaking the API.

Change-Id: I4200b6600df8e56fe98cf1580a13a65cb25ec646
Signed-off-by: Rom Lemarchand <romlem@google.com>