service: Handle configure_mtu_cb, search_complete_cb and search_result_cb events

This patch adds HAL wrappers for those three events in preparation to
triggering MTU exchange and service discovery.

search_result_cb will not be propagated above HAL, new method will be
implemented to get GATT database content when it's ready.

Change-Id: Id25699879923ff64c8bdb8942581ea32c2390dac