Fix missing GATT Characteristic from last service

GATT Service is contained between start and end handle. If
characteristic definition is at end handle, and it's value definition is
after end handle, it will not be properly discovered. That's because we
use value_handle instead of attribute_handle to identify
characteristics.

As a workaround, increase service boundary if value is defined after
it's definition.

Bug: 29253825
Change-Id: Ib145aea4f5cf38a1fbb977c301136e16f8f900f7