Add new HAL method get_gatt_db

Currently getting GATT database from HAL requires multiple calls
and iterating over each element. This is long, unnecessary
complicated process, error prone process. This patch adds new
method, get_gatt_db that can be used to grab whole GATT database
for remote device just in one call.

Change-Id: Ib5d88fbc28d09d5c191e7c0152ca61d6a34f7a81