Remember what stage of service discovery are we during bonding

This will be useful to decide if we are done before sending UUIDS to
uper layers, and to ensure discovery is started at most just once for
each transport

Bug: 248340112
Test: pair with LE, Classic, and dual mode devices
Change-Id: I1e91469fce49c0f544c979587f7ffb8ecac3b076