[bt][gap] HACK: discover services after pairing

bt-host currently initiates pairing as soon as a LE connection is
created. This is temporary until support is added to initiate
pairing in response to a service request.

This patch removes a race between service discovery and pairing by
performing the former only after the pairing procedure concludes.
Services are discovered regardless of the pairing result.

Change-Id: Ia0cb27172ca7e105cb35d9a74c19e348b3e35086
TODO: bug number
TODO: tests
7 files changed