Bluetooth: tester: Return pre defined db offset on start server

Some IUTs may have predefined attributes in db which aren't take into
consideration while building via tester database. BTP clients should
know about such attribute handle offset change. After start server
we're sure that none of attributes will be added since zephyr db is
constant.

Change-Id: Ia2c6edaa7a2f3ca4a9c88fe00122c808a529dadb
Signed-off-by: Grzegorz Kolodziejczyk <grzegorz.kolodziejczyk@tieto.com>