Use attribute handle instead of id in GATT cache

Use attribute handle instead of id in GATT cache

Currently we generate an id for each GATT attribute. Those ids
are used to differentiate between attributes that have the same
UUID. We need to get rid of those ids because it won't work with
partial GATT database updates due to "service change"
notifications. Instead, attribute handles, which are unique
across the whole GATT database will be used.

This is first patch in series to achieve that. It gets rid
of id generation, and just assigns handle in place of ids inside
of gatt cache. Upcoming patches will change the GATT cache API
and eventually get rid of "id" completly.

Bug: 27455533
Change-Id: I74c10872ecc223329b8ac7fef4085963bf272624