Do not mark an LE link as authenticated after Just Works pairing

[deadcaf3]

Summary

Connection.authenticated was set to true after every pairing, and again
whenever LE encryption was enabled. The GATT server's
READ_REQUIRES_AUTHENTICATION and WRITE_REQUIRES_AUTHENTICATION checks read
that flag, so a peer that paired with Just Works could read and write those
attributes. With the default PairingDelegate, Just Works needs no user
interaction. The keys stored for the same pairing were already marked
authenticated: false.

The flag is now derived from the key that the link is encrypted with.

Behavior

Link encrypted by	authenticated before	authenticated now
Just Works pairing	true	false
Passkey, numeric comparison or OOB pairing	true	true
Stored LTK from a Just Works pairing	true	false
Stored LTK from an authenticated pairing	true	true

Notes for review

• After pairing, Device.on_pairing takes the flag from the keys it is given,
  which smp.Session.on_pairing already marks according to the pairing method.
  smp.py and the on_pairing signature are not changed.
• When the link is encrypted with an LTK from the key store, the flags of that
  key are recorded in a new Connection.ltk_security when the key is given to
  the controller (Device.get_long_term_key as a peripheral, Device.encrypt
  as a central), and applied when the Encryption Change event arrives.
• Connection.ltk_security is cleared when a pairing starts and when
  encryption fails, so that the flags of a stored key that did not encrypt the
  link are not applied to a later encryption.
• Device.encrypt checks the role before it looks up the key, so a call on a
  peripheral does not record the flags of a key that is never used.
• sc is set the same way. Before, it was set to true on any LE encryption
  change, including with a legacy LTK.
• Classic links are not changed.
• The reconnection tests only cover Secure Connections bonds. Re-encrypting
  with a legacy bond disconnects the link with the virtual controller, with or
  without this change.

Compatibility

• Servers with attributes that require authentication now answer Insufficient
  Authentication to a peer that paired with Just Works. In this repository that
  applies to apps/pair.py and
  examples/run_gatt_server_with_pairing_delegate.py.
• During a pairing, authenticated and sc are no longer set when encryption
  starts. They are set when the pairing completes, before the pairing event.

Testing

Format, lint and the test suite pass, and mypy reports nothing in the changed
files. Three tests (8 cases) are added to tests/self_test.py; the Just Works
cases fail without the change.

[zxzxwu]

LGTM, thanks! Left two minor non-blocking nits inline.

Also a tiny heads-up: a few lines in the PR description appear to have been truncated when pasting (e.g. under "Notes for review" and "Compatibility").

[zxzxwu]

Nit: Consider moving the if connection.role != hci.Role.CENTRAL: check (just below) before looking up the key / setting connection.ltk_security, so a failed encrypt() call on a peripheral doesn't leave connection.ltk_security set.

[deadcaf3]

e2e75c1 gets the roll check run first before key store lookup; peripheral never records key's flags.

[zxzxwu]

Nit: Should we also reset connection.ltk_security = None in on_connection_encryption_failure below so stale security flags aren't left on the connection if encryption with a stored LTK fails?

[deadcaf3]

e2e75c1 also clears ltk_security in on_connection_encryption_failure.