[bt][l2cap] Add API to upgrade security of a link
The L2CAP system now has a sense of "link security properties". This
will allow the channel creation state machine and service-level users to
request security upgrades. Channel users can now also query the link
security properties directly using the channel.
The actual link security upgrade is not performed by the L2CAP layer but
rather delegated to the layer responsible for registering the link. This
will be done by the GAP layer once this is exposed via data::Domain. It
is also the responsibility of the registering layer (i.e. GAP) to keep
the link security level stored by L2CAP up-to-date.
NET-1151
NET-881 #done
Test: bt-host-unittests
Change-Id: If975da88457919c999fe798211378bdad6dd6c62
16 files changed
