GD Security: Improve link encryption storage

Encryption is a property of link.  We update it when we receive
encryption change event, and then continue L2cap security Enforce()
workflow.

For RFCOMM and OPP, we just need encryption (remote device is paired),
without authenticated link key or MITM.

Tag: #gd-refactor
Bug: 141555841
Test: cert/run --host
Change-Id: I42c7ccd3b8abac1acdef10833da574e4f798ef20