Changes to allow LE key upgrade

Use Case: Certifiation test cases require the LE key upgrade
to happen when the current MITM level of the device has
been upgraded. In that case, a repairing is performed,
leading to more authenticated LTK.

Test Case: TP/SEC/AUT/BV-17-C

Steps:
1. Perform LE pairing between two devices using no MITM.
2. Change the IO capability of the devices, so that a
repair would involve authentication.
3. Reconnect the devices and expect key upgrade.

Failure:
No key upgrade is seen at reconnection and the devices
continue to use the old key.

Fix:
Added changes to re-calulate the requirement of key
upgrade when the LE channel is already encrypted.

Bug: 27852645
Change-Id: If62f334dcf298b2ce9840e95035b4398cb951c84