floss: Add check to determine if read encryption key size is supported

On some laptops, pairing a bluetooth headset can fail. The ChromeOS
log at /var/log/bluetooth.log shows that this is due to using
`READ_ENCRYPTION_KEY_SIZE` when it is not supported: there is a line
with `UNKNOWN_HCI_COMMAND opcode:0x1408 (READ_ENCRYPTION_KEY_SIZE)`.
To remedy this, add a check before trying to read the encryption key
size on receiving a encryption change event from the controller.

Bug: 273316924
Bug: 273796661
Bug: 274092923
Tag: #floss
Test: Verify that bluetooth headset can be paired on a Dell Latitude
3440, Dell Latitude 3340, and HP Probook 450 G1

Change-Id: I02cb0f938be4430923b4ad8091ef05ae69336c87