Fix race condition around Le Start Encryption handling.

Fix race condition around Le Start Encryption handling.

When sending Le Start Encryption, it's possible that at the same exact
moment the connection was disconnected, and there is an incoming
"Disconnection Complete Event". The controller would send error 0x02
"unknown connection identifier" in such case. This can happen
sporadically when reconnecting device multiple times.

Currently, we don't handle this error code. This lead to the timeout
when waiting for "Encryption Complete", and unbonding of the device.

From now on, if 0x02 error code is returned, we'll cancel current SMP
operation, which is the "LE Start Encryption". This will stop the SMP
timer, and make sure the bond to remote device is not lost.

Test: Manual test reconnecting with HID device multiple time in a row,
causing the link drop right after connection, by removing battery, or
closing the shield box.
Bug: 113652889
Change-Id: I2ff9c13dbc8e7b71505908996e26b89fa1ea6a42