GATT Server refactoring (4/4)

Right now in order to add gatt service, we need to add it piece by
piece - each characteristic and descriptor separately. This causes lots
of concurrency errors in higher layers. Instead accept whole service
definition and start whole service at once.

Also, use handles to identify GATT server attributes. It's already done
on clinet side.

Bug: 27999121
Change-Id: I42c796809423d116fe6755ef8b1c40da270427a5