Bluetooth: SDP: Refactor bt_sdp_discover API

Renames and refactors helper get_client_session to sdp_client_get_session
to follow existing naming convention and adds second helper creating
new SDP client session. Then simplifies using them the API implementation.

Jira: ZEP-1112

Change-Id: I6b919f521e6665a7117fa06208b3fa2ae5f77fda
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>