L2cap: Add SecurityModuleInterface

Before we send or respond to a ConnectionRequest, we ask SecurityManager
to enforce that we have sufficient link layer security.

SecurityManager needs to implement SecurityModuleInterface, and inject
the implementation to L2cap module. L2cap provides a default
implementation which rejects all connection which requires link key.

Test: cert/run --host
Bug: 141555841
Tag: #gd-refactor
Change-Id: I2f6b841ddc34e4bd9611b6d0f06657cc342c6612