Refactor InputHost and InputDriver

The host implementation of the HAL interface is largely done by the
InputDriver, so have it implement the opaque input_host_t type rather
than the InputHost itself. This allows the HAL interface to cast the
input_host_t pointer to an InputDriver pointer in order to implement
the host functionality.

Change-Id: I72de1e90eb6ee5e346c15707b8a6b793005bbccb