LE: Check encryption state before starting encryption

Previously, the btif layer kept track of when a link was encrypted using
an internal list to track encryptino callbacks. This change switches to
querying the stack for the actual encryption state, since the btif layer
may not receive all encryption related callbacks.

Bug: BLTH02382147
Change-Id: I426b87d77e9a18531c2d42733a7e736a73bb116f